Possible causes for a missed period-- not pregnancy.

So, I missed my period the month of March. But, the last time I was sexually active was in November. I've had my period every month since then except now. I also haven't gained abnormal amounts of weight and haven't had any symptoms that can't be explained by other causes. I highly doubt I'm pregnant but I'm still worried. I mean, February is a short month. My work schedule changed from PM to AM throwing off my sleep schedule and I had to be put on two different antibiotics twice right before I was supposed to get my period. Could it just be a mix of those things?