I started bleeding between cycles is this serious?

I am a 40yr old who started bleeding in between my periods. I do not have ins., and I was wondering if this might be something serious. It starts around the time of ovulation and last for around 5 days. As with my reg. period it is very heavy and I have a lot of pain.This has been going on for 5mths. I was not that concerned but last month it started again around my ovulation time and did not stop. I started my period on the 12,ended the 18th, started bleeding agai on the 26th and did not stop til 18th of the next mth. This mth has been ok, so I dont know if I should be worried. Has anybody had this happen. Cancer keeps popping up in my head. I need some answers and advice PLEASE HELP,