Need a quick answer! Should I go in tonight and get this checked out or hold off?

26 weeks pregnant, having menstrual-like cramping for over 24 hrs now. I thought it might be constipation or something so I've not been too worried. However, I kept waking up all night last night with cramps, and they've continued today despite having a decent bowel movement. I've also been having a lower back ache this whole time. I called my doctor an hr had a half ago (she just happened to be on call) and she knows that I'm prone to GI problems with this pregnancy and told me to take some Gas-x and and an IB-Profen and give it two hrs and if nothing improves, come in to the hospital for an NST. Well it's coming up on 2 hrs, and things have improved some, the cramping isn't as painful, but my stomach is still tightening up like one every 10-15 mins. I feel like I could just go to bed, but I don't want to be stupid if this could be a sign of preterm labor. Thoughts??