I'm 20+3 with twins on the way, first pregnancy. I've noticed this a bit over the last couple of days, but it's been happening more today - my bump keeps really tightening up (sort of like something's gone into spasm, but not painful) for about 20-30 secs and then releasing again. When it happens I get all sweaty and afterwards I need to pee straight away! I'm not worried because it doesn't really feel like anything is wrong, but it's the weirdest sensation.
Has anyone else had this around the 20 week mark??