Braxton Hicks in the second trimester is actually more common than people think, it can happen from around 20 weeks onwards, especially in subsequent pregnancies, so you're not alone in experiencing it earlier this time. The key thing I've heard is that true Braxton Hicks usually eases off when you rest, change position, or drink water, whereas real contractions tend to stay regular and don't let up no matter what you do, so that's a useful thing to watch for. The back pain alongside the tightening is worth monitoring closely though, and since your doctor has already prescribed medication to reduce contractions and flagged the preterm risk, that tells me he's taking it seriously, which is actually reassuring in a way. One mum I know was told to track whether the tightenings come more than four times in an hour, and to head in if they do, rather than waiting it out at home. You're already in good hands with your doctor keeping an eye on things, but if anything feels different or the frequency picks up, do loop back with your OB or specialist rather than wait for the next scheduled visit.