true or false labor?

Hey guys I'm a little confused, I have been having what I think are contractions It's like I feel like my cervix area tightens up really tight and then it releases, and I feel like below my belly button balls up as if The baby is arching her back out every now and then. And it hasent been hurting too bad but it's starting too and my belly  is super firm when I get these pains and I have been timein them they are about every 5 mins and are lasting a little over 30 second when I get them.I'm 36 weeks and 3 days. Have any one had contractions that were kinda like thisI lost my mucus plug last week it was actually brownesh and it was over the course of a few day