3-4 months late and 1 negative test but possible signs

I am 19 and I have always had very regular periods (I could predict them to the day). I started using the pill a year ago but I was careless and missed it every few days (but still was very regular). so about 4 months ago I stopped taking it altogether. sometime around then we used a condom but it kind of came off too soon. for months (and especially recently) I have been feeling extremely tired, some days I sleep 15-16 hours if Im not woken up. Also I have gained about 15 pounds since then, mostly around my stomach (unfortunately). but, to make it even more complicated I am on medicine for bipolar disorder although it says I should be losing weight rather than gaining on the side effects. what should I do?? could I still possibly be pregnant?