very tmi with picture (TMI)

I have a little bit of dark ish blood on my undies... It's be happening for a couple weeks now and I didn't think anything of it because I thought it was my new jeans rubbing off on my panties at first but I wore a dress all day and white panties and now know it's a bit of older blood. 31 weeks pregnant. Any ideas or suggestions. (Brand new underwear, first time wearing them. No blood when I wipe).