Pain/discomfort--is this normal?

I am 19 weeks pregnant with twin girls... During the last week I have become increasingly uncomfortable. Already I can barely bend over, and my stomach (mostly the top) is KILLING me.  The pain is almost a tightening feeling all around the top and sides, but not like the ligament pain I experienced with my daughter 3 years ago.  Is it normal to be in so much discomfort this early? I woke up this morning feeling fine, and by mid-morning it was back to being painful. I am paranoid I am dehydrated and will begin cramping soon (the only stomach pain I felt last time ended up sending me to the hospital with PTL).  Please tell me it's just that there's two kids in there and getting tight...

 Thank you.