gas pain, cramps, contractions??

I'm 16 weeks with twins and have been really uncomfortable the last few hours. I had really thick discharge today and Dr didn't seem too concerned but said take it easy (just talked over the phone). for the last 30 mins I've been having lower back pain that comes and goes and kind of wraps around my belly. It doesn't feel like my belly is hardening. I sometimes get bad gas pain but this just feels different. How concerned should I be? This is my first pregnancy, and I feel like I'm so worried am the time .