Pain!

Anyone know why I have horrific upper right back pain and pain in my right rib cage all at the same time it hurts horribly after sitting on the couch for a while trying to get my other two kids to sleep it always happens at night the most and sometimes during the day I'm 36 weeks and I had some contractions last night but they were not really constant just a couple here and there and I did lose my mucous plug the day before yesterday at the dr when she checked me and I was also 2cm dialated I am almost positive I've dialated more as well