Se eu levantar muito rápido, fico tonto.

Because after se in a real future condition, Portuguese normally uses the future subjunctive.

So in Se eu levantar muito rápido..., levantar is not the infinitive here in function, even though it looks the same. It is the future subjunctive form.

This is very common in Portuguese:

• Se eu levantar... = if I get up...
• Se eu fizer... = if I do...
• Se eu tiver... = if I have...

With regular -ar verbs, the future subjunctive often looks identical to the infinitive, which is why it can feel confusing at first.

Here, the present tense expresses a general result or habitual consequence.

So the sentence means something like: whenever this condition happens, that is what happens to me.

Portuguese often uses the present this way:

• Se eu como muito, passo mal.
• Se eu durmo pouco, fico cansado.

If you wanted to focus on one specific future occasion, you could also hear:

• Se eu levantar muito rápido, vou ficar tonto.
• Se eu levantar muito rápido, ficarei tonto.

But fico tonto sounds very natural for a usual, repeated result.

In this sentence, levantar means to get up or to stand up, not to lift something.

That verb can have different meanings depending on context:

• levantar a caixa = lift the box
• levantar da cama / da cadeira = get up from bed / from the chair

Here it refers to your body position changing too quickly.

Se eu me levantar muito rápido is also correct and very natural.

In Brazilian Portuguese, with some everyday verbs like levantar, people often omit the reflexive pronoun when the meaning is already clear:

• Vou levantar.
• Eu me levanto cedo.
• Eu levanto cedo.

So both are possible:

• Se eu levantar muito rápido, fico tonto.
• Se eu me levantar muito rápido, fico tonto.

The version with me can sound a little clearer or a little more explicit, but the version without it is very common in Brazil.

Yes, rápido is very commonly used this way in Brazilian Portuguese, especially in everyday speech.

Even though rapidamente is the more formally adverbial form, Brazilians often use the adjective form adverbially:

• Fala rápido.
• Anda rápido.
• Levanta rápido.

So all of these are possible:

• muito rápido — very quickly / too quickly
• muito rapidamente — more formal
• muito depressa — also very natural

In normal conversation, muito rápido sounds completely natural.

Literally, it is very quickly, but in context it often implies too quickly.

Since the result is negative — fico tonto — listeners naturally understand that the speed is excessive.

If you want to make too more explicit, you could say:

• rápido demais
• depressa demais

For example:

• Se eu levantar rápido demais, fico tonto.

That makes the idea of too quickly even clearer.

Because adjectives in Portuguese usually agree with the gender of the person being described.

So:

• fico tonto — if the speaker is male
• fico tonta — if the speaker is female

The same sentence changes like this:

• Se eu levantar muito rápido, fico tonto.
• Se eu levantar muito rápido, fico tonta.

Ficar + adjective is a very common pattern in Portuguese meaning to become or to get + adjective.

So:

• ficar tonto = become dizzy / get dizzy
• ficar cansado = get tired
• ficar nervoso = get nervous

This is one of the most useful verb patterns in Portuguese. Instead of using a separate verb for become, Portuguese often uses ficar.

In the first clause, eu is very helpful and often expected, because levantar could correspond to several subjects in the future subjunctive:

• se eu levantar
• se você levantar
• se ele levantar

Without the pronoun, the subject would be unclear.

In the second clause, eu is less necessary because fico already clearly marks first person singular. So:

• Se eu levantar muito rápido, fico tonto. — perfectly natural
• Se eu levantar muito rápido, eu fico tonto. — also correct, with extra emphasis

Yes, the comma is standard and recommended here.

The sentence begins with a conditional clause:

• Se eu levantar muito rápido

Then comes the main clause:

• fico tonto

Portuguese normally separates that initial conditional clause with a comma:

• Se eu levantar muito rápido, fico tonto.

You may sometimes see informal writing without it, but the comma is the correct standard punctuation.

Se means if, so it presents the situation as a condition.

• Se eu levantar muito rápido, fico tonto.
  = If I get up too quickly, I get dizzy.

Quando means when, which sounds more certain or more like whenever.

Compare:

• Quando eu levanto muito rápido, fico tonto.
  = When/whenever I get up too quickly, I get dizzy.

That version is also natural, but it presents the event a bit differently.
Se focuses on the condition.
Quando focuses more on the repeated occasion.

Yes. Some very natural alternatives are:

• Se eu me levantar muito rápido, fico tonto.
• Se eu levantar rápido demais, fico tonto.
• Quando eu levanto muito rápido, fico tonto.
• Se eu levantar muito rápido, eu fico tonto.

They are all close in meaning, but each has a slightly different tone:

• me levantar = more explicitly reflexive
• rápido demais = more clearly too quickly
• quando = more like whenever
• repeating eu = more emphasis

The original sentence is already perfectly natural Brazilian Portuguese.