kind of a weird question.

I'm 33 weeks pregnant and for the past couple days I've had a weird burning up around my cervix. I don't have an infection of any kind and it get SOOOO much worse with sex. I've been test for all kinds of vaginal infections, UTIs, STDs you name it and everything is negative. Has anyone experienced this? What is it? Is it my cervix trying to soften? I just don't get it but it hurts!