Why My Period Is Not Coming?

i am a 19 year old girl.i hve irregular periods but then i was diagnosed with thyroid so i have the medicine daily prescribed for thyroid.after starting thyroid medicine my periods started(which had stopped months ago...due to stress)..i have been having this medicine since june 2011. in august i had my last period..in september i had protected sex with my boyfriend..and since then my periods didnt come..the condom didnt break and there isnt any symptom of pregnancy...and i had home pregnancy test and it showed negative..and also...i have under lot of stress..my periods hasnt arrived yet..plzzz help!!