Pressure after urination?

I'm 18 weeks and 4 days and every night and every morning when I empty my bladder I get bad gas cramps, but there's no gas! When I lay back down it goes away within a few minutes, but it's so uncomfortable! Is it just gas that I'm not passing? Or not noticing that I'm passing? Or braxton hicks? Or round ligament? Haha I'm a FTM and I just have no clue what it could be! I'm not worried about it, just curious! Thank you!!