Prekoračila sam planiranu potrošnju mobilnih podataka.

Because Serbian forms the past tense with:

• the l-participle: prekoračila
• the auxiliary biti in the present tense: sam

So Prekoračila sam is the normal past-tense structure for I exceeded / I have exceeded.

A useful pattern is:

• uradio sam = I did
• videla sam = I saw
• prekoračila sam = I exceeded

In Serbian, this past tense can correspond to either English simple past or present perfect, depending on context.

Because prekoračila is the feminine singular form.

The speaker is female, so the participle agrees with the speaker:

• Prekoračila sam = said by a woman
• Prekoračio sam = said by a man

This is very common in Serbian past tense: the verb form shows the speaker’s gender.

Serbian often leaves out subject pronouns when they are already clear from the verb form and context.

So:

• Prekoračila sam... already means I exceeded...
• Ja sam prekoračila... is also possible, but ja adds emphasis, like I specifically

English usually needs the pronoun, but Serbian often does not.

Because sam is a clitic: a short unstressed word that normally cannot stand at the beginning of a sentence or clause.

So:

• Prekoračila sam... = correct
• Ja sam prekoračila... = correct
• Sam prekoračila... = not correct as a normal standalone sentence

Clitics in Serbian usually come in the second position of the clause.

It is the accusative singular, because it is the direct object of the verb prekoračiti.

The noun is:

• potrošnja = consumption, usage

Since potrošnja is feminine, its accusative singular is:

• potrošnju

The adjective must agree with the noun in gender, number, and case:

• nominative: planirana potrošnja
• accusative: planiranu potrošnju

So planiranu potrošnju means the object being exceeded.

Because mobilnih podataka depends on potrošnja.

Serbian often uses the genitive after nouns that express quantity, amount, or consumption:

• potrošnja struje = consumption of electricity
• potrošnja goriva = fuel consumption
• potrošnja mobilnih podataka = consumption of mobile data

So here the structure is basically:

• potrošnja čega? = consumption of what?
• mobilnih podataka

That is why podataka is in the genitive plural.

In Serbian, podatak is a normal countable noun meaning a piece of data / a datum, and podaci is its plural.

In everyday Serbian, mobile data is commonly expressed with the plural:

• mobilni podaci
• genitive plural: mobilnih podataka

So although English often treats data as a mass noun, Serbian usually treats podaci as a plural noun in this kind of phrase.

Literally, planiranu means planned.

So the sentence literally refers to the planned mobile data usage.

However, in real-life telecom contexts, English speakers often mean something like:

• my data limit
• my data allowance
• the included amount of data

In Serbian, planiranu potrošnju is understandable, but depending on context, people might more naturally say things like:

• Prekoračila sam limit za mobilne podatke.
• Prekoračila sam dozvoljenu potrošnju mobilnih podataka.
• Potrošila sam više mobilnih podataka nego što je planirano.

So planiranu is grammatically fine, but its exact nuance depends on what kind of limit or plan you mean.

Prekoračiti is perfective.

That means it presents the action as a completed whole: you went over the limit.

Its imperfective partner is prekoračivati, which would be used for repeated, habitual, or ongoing situations.

Compare:

• Prekoračila sam limit. = I exceeded the limit.
• Često sam prekoračivala limit. = I often used to exceed the limit.

In your sentence, the perfective verb fits because this is a completed result.

Yes. The given sentence is grammatical and understandable, but Serbian offers several natural alternatives depending on style.

Some common options are:

• Prekoračila sam potrošnju mobilnih podataka.
• Prekoračila sam limit za mobilne podatke.
• Prešla sam limit za mobilne podatke.
• Potrošila sam više mobilnih podataka nego što je planirano.

Very roughly:

• prekoračila sam sounds a bit more formal or technical
• prešla sam limit sounds more everyday and conversational

So the original sentence is fine, but not the only natural way to say it.