I think a trip to the doctor is probably called for - the causes could be numerous; some serious - other's not. You don't seem to mention fevers, cramping, but you did say heavy bleeding - which warrants a doctors appointment. Question is, how heavy?
There are so many causes that it's hard to pinpoint - anything from hemorrhoids, constipation, digestive diseases - the list goes on.
If it's been happening regularly - it's worth the effort getting things checked out. I suggest making a list of ANY symptoms you are experiencing that are "other than 100% normal".
A little effort will give you peace of mind ...
Hi, look then you now know the answer, you may just have give up weight lifting for a couple of months to get your self better, its a strain that you have, and when you restart start low and build up and just hope you have then got over it.
Good Luck