Help!

Hey guys. I am almost 37 weeks. For the past 3 days I have had constant period like cramps that are pretty strong. Every now and then I have a stabbing pain in the bottom of my belly and lots of pressure on my rectum. I don't really know how or if I should be timing any of this. The period like cramps don't really ever go away. I also have the cramps around in my back. I have been on a heating pad all day. When I was checked last about 3 weeks ago I was 70% effaced. When should I be concerned?