I can feel my heartbeating quite fast in my stomach and i get twinges every now and again, wot does this mean?

im also on chemotherapy and im on tablets to stop my periods whilst on the chemo, iv not had with a boy or done anything with a boy but for some strange reason iv got it in my head that im pregnant and of course i dont see how i can be! my last period was late by 2 weeks and was really heavy due to my medical condition, but why can i feel my heart beating really fast all the time and iv put on weight, im getting worried!!