Poslije toga sam otišla u ribarnicu, jer mi se večeras jede riba, a ne meso.

Croatian often leaves out subject pronouns when they are not needed. The verb form already shows the person:

• sam otišla = I went / I have gone

So ja is usually omitted unless you want emphasis or contrast:

• Ja sam otišla u ribarnicu, a on nije. = I went to the fish shop, but he didn’t.

In a neutral sentence, leaving ja out sounds natural.

Because otišla is the feminine singular past participle. It tells you the speaker is female.

Compare:

• otišao = a male speaker went
• otišla = a female speaker went
• otišli = a mixed/male plural group went
• otišle = an all-female plural group went

So:

• sam otišla = I went said by a woman
• sam otišao = I went said by a man