Describe what contractions feel like

I'm 37w3d and I have a fear that I won't know when it's time to go to the hospital. I had Braxton hicks for the first time all last week. On Tuesday I was 50% effaced. Today I've just been feeling really crappy with period cramps and lower back pain on and off all day. If my water doesn't break on its own (which I know doesn't happen often) then I'm afraid I'll be one who shows up way too early or too late!