I'm preggo w/my first so I have no experience to share, but I'm pretty sure cramping is very normal at the end of pregnancy
as your body prepares for labor/birth. I've had occassional menstrual
too and I'm not as far along as you. My doctor equates it to either BH, normal uterine cramping, or constipation or diarrea. Do you have an appointment soon? Definitely mention it next time you see your doctor. If it's going to be awhile, I'd call if it gets any worse.