question about cramping 36 weeks

I'm 36 weeks and 5 days and the past week if I sit down I get cramping feeling in my bottom and under my belly it isn't a contraction cause my stomach doesn't tighten and it comes and goes it's not consistsntconly when I sit down for a little but it's a cramping in my lower belly and bottom when it first starts it kinda uncomfortable just wanted to know if anyone was feeling or having same symptoms or knew what hey could be