What Causes Stomach Pain And Cramping After Delivery?

I am 30, 5"6 and approx 10.5 stone. I have stomach pains that sometimes can be of a sharp nature but largley cramping, like when I am on my period. They are very low down in my abdomen and can last for days/weeks at a time. My appetite is normally, and I have regular bowel movement once/twice a day. I do feel a little tired and have a lot of wind. I have had this for a couple of years, but noticed that when I was pregnant this pain stopped during my second/third trimester but has returned 3 months after giving birth. What could this be?