So I haven't had my period in two months, I haven't taken a pregnancy test, but I also haven't had sex since my last period... so I should be on my period right?. I was on birth control but I got off it because it was making me gain a lot of wieght... so now that I sort of feel back on track with my wieght my period won't come and I have no idea what is going on!! Is there a chance of some sort of cancer causeing this??

You need to take a HPT using first morning urine. It's possible that, if you are pregnant, you got pregnant too late in your last cycle. Therefore,you had a period last month, but not this month.

I actually haven't been sexually active in about a year!! I messed around once and there was slight penetration but I stopped it before anything happened so I have no idea what to do!

How long ago did you stop the birth control? If it was fairly recent, it may be your body adjusting to that. Since this is in the child/teen section I assume you are a teen... cycles can also sometimes be a bit off during those years, even if you have been regular up until now.
As J_9 suggested, try a pregnancy test if the slight penetration was within the last couple of months. While not likely, depending on the situation, pregnancy might be possible and best to rule it out if it fits the time frame.
Odds are very unlikely regarding cancer... I know when things are not the norm it is easy to worry about those possibilities. Rare in someone young and usually one would be more likely to see unusual bleeding, as in an increase.
You could always go see your doctor to be checked so that you can have some peace of mind.