Thanks for discussing your problem to us.
The easiest way to relieve your doubts about pregnancy
is to have one blood pregnancy test. If this is negative you can't be pregnant. Your recent weight gain may have a role in the breakthrough bleeding
you are having. So, consider loosing weight.
You should also see a Gynecologist to exclude local causes of this bleeding & cramping.
If you are afraid of weight gain you may try other non-hormonal
methods of contraception
like intra- uterine devices.
Wish you good health.