Nakon dvije tablete i malo odmora počela sam se oporavljati.

Croatian past tense is usually made with jesam as an auxiliary plus a past participle.

So:

• počela = past participle of početi (to begin), feminine singular
• sam = I am, used here as the auxiliary for I have / I did in the past tense pattern

Together, počela sam means I began or I started.

A masculine speaker would say počeo sam.

Because the speaker is female.

In Croatian, the past participle agrees with the subject’s gender and number:

• počeo sam = I began, said by a man
• počela sam = I began, said by a woman
• počeli smo = we began, masculine/mixed group
• počele smo = we began, all-female group

So počela sam tells you the speaker is a woman.

That is the normal Croatian word order for this kind of sentence.

Croatian clitics, such as sam and se, usually come very early in the clause, often after the first stressed element. In practice, with the past tense, počela sam is the natural order.

So this sounds normal:

• Počela sam se oporavljati.

Putting sam somewhere else is usually less natural here.

Se is the reflexive particle. It is part of the verb oporavljati se, which means to recover, to get better, or to recuperate.

This is important because:

• oporavljati se = to recover oneself, to recover
• oporavljati without se usually means to restore or to improve something

For example:

• Oporavljam se. = I am recovering.
• Oporavljaju ekonomiju. = They are restoring/improving the economy.

So in your sentence, se is not optional; it belongs to the verb.

Because after početi (to begin/start), Croatian normally uses an imperfective verb to describe the beginning of an ongoing process.

• oporavljati se = imperfective, to be recovering / to recover over time
• oporaviti se = perfective, to recover completely / to get recovered

So:

• počela sam se oporavljati = I started to recover

That sounds natural because recovery is a process. Croatian usually prefers početi + imperfective infinitive.

Nakon takes the genitive case.

That is why the words after it are in genitive forms, for example:

• nakon odmora = after the rest
• nakon ručka = after lunch
• nakon bolesti = after the illness

In your sentence, nakon governs the whole phrase dvije tablete i malo odmora.

Because malo here means a little (of) or some, and after this kind of quantity word Croatian uses the genitive.

So:

• malo vode = a little water
• malo vremena = a little time
• malo odmora = a little rest

Also, this malo is not the adjective small. Here it is a quantity word meaning a little / some.

This is a very common learner question.

With the number 2, Croatian uses:

• dva with masculine and neuter nouns
• dvije with feminine nouns

Since tableta is feminine, you get dvije.

Now for the case issue:

• In everyday Croatian, nakon dvije tablete is very common and natural.
• In more careful or formal standard Croatian, many grammars prefer nakon dviju tableta after a genitive-governing preposition like nakon.

So:

• nakon dvije tablete = very common in real speech
• nakon dviju tableta = more strictly case-marked / more formal

A learner should definitely recognize both.

Because with numbers like 2, 3, 4, Croatian nouns often appear in a special counted form.

For many feminine nouns, that form looks exactly like the nominative plural:

• jedna tableta
• dvije tablete
• tri tablete
• četiri tablete

So even though it looks plural, it is part of the normal pattern used after these numbers.

This is one reason number phrases can feel tricky in Croatian.

Croatian often expresses this idea more simply with nakon plus a noun phrase.

So:

• Nakon dvije tablete i malo odmora...
  literally = After two pills and a little rest...

English often prefers a more explicit phrasing like after taking two pills, but Croatian can leave taking understood from context.

This is natural and idiomatic.

Yes. Nakon and poslije can both mean after.

So this is also possible:

• Poslije dvije tablete i malo odmora počela sam se oporavljati.

That said:

• nakon often sounds a bit more neutral or formal
• poslije is also very common in everyday speech

Both are good Croatian.