Preterm labor?

I know this is a difficult subject to talk about, but has anyone experienced pre term labor? If so, what did contractions feel like? This is my second pregnancy and I am aware of what Braxton Hicks feel like. I am currently only 18 weeks, but woke up multiple times in the night with my uterus contracting and it wrapping into the left side of my back. I don’t remember Braxton Hicks being this intense so early on, or feeling them as much as I did in my back. I haven’t had any since I’ve woken up, but am looking for some insight.