Mitä tein eilen?

Mitä tein eilen (No question mark, because it is a title, not a real question.)

Sää oli kaunis eilen, mutta opiskelin olin yliopistolla kirjoitamassa ja minulla on flunssa. Tuntui kauhealta.

(If you say "opiskelin" it's more to say that you are studying in general, not that you studied for a particular thing. And since you say afterwards "kirjoittamassa", correct form to use here is "olin kirjoittamassa" (I was writing). And "yliopistolla instead of yliopistossa, because you were at the uni, not in.)