Braxton Hicks Vs Actual Contractions

Can someone please explain to me the difference between BH & actual contractions? I'm 35w3d &  I've been having horrible lower back pain the last three days, accompanied by twinges of pain, almost period like cramps. At first they were just uncomfortable, but now they are becoming more frequent and are forceful enough to stop my in my tracks. I don't see my OB until the 7th & I don't know how concerned I should be. The only time I feel remotely comfortable is laying in a warm bath. 😩