Pain at top of uterus...

Shayla • Happily married SAHM to Mason Paul 5.30.12, Ryan Thomas 9.3.15
I am 37+4 today. Baby Ryan is head down, about 2 cm dilated, cervix is "super soft". No contractions or anything yet aside from today being day 5 of constant lower back cramps. Doc said she'd be concerned if I was dilating but I'm not so just sucking it up for now. 
Yesterday I was running the vacuum and after I was done I noticed a spot right at the top of my uterus that feels like it's being pulled on, toward the inside. It's decently painful, but I haven't had any spotting or anything else to cause concern. Does anyone have this or have any experience with this? Thanks!