39 weeks and have question on contractions??? Please help?

Ok I have been gettin cramps strong on and off for about an hour the cramps are strong (menstral type) that I feel in to my knees at times. Now all of a sudden i am getting discomfort in my lower back now it could be just cause I have been sittin at this computer in the chair for a while LOL. what ae contractions like??? All I have heard is if your having them you will know. they hurt but has anyone had them where they were minor discomfort and if so what happened. TY I am first time mom. my first due date was nov 10 from my cycle dating (last period was feb 2) but then the baby measured small and it was changed to dec 2. LOL if you can relate to this how much longer do i have just for fun>????