How would you know if you were going into pre term labor

I am 34 weeks and 3 days right now and I'm having a lot of back pain and trouble breathing. There is a really weird thumping sensation in my lower back area but it isn't the baby kicking. I can't tell what it is. Because I am young, glow says I am at risk for pre term labor and I'm afraid it's happening now. I'm not really sure how to tell or when to go to the hospital.