New symptom??

So I think I'm experiencing a new symptom today, I'm having mo/di twins (gender not yet known) 12 weeks tomorrow and today I've been experiencing a lot of abdominal muscle soreness, this is my first pregnancy so this is all new and it being a twin pregnancy is throwing me off even more, this the muscle soreness common? I know that once I start getting bigger everything stretches, but I don't know when to expect it to start, am I going crazy? Crazy-er?