Hi
Yes it probably is your uterus, mine was about there are 21 weeks and I've had some bad stretching pains, just to warn you my stretchy pains have spread to my ribs! I also get sore spots where the babies press up against the uterus wall too, so the hard sore bits could be babies! I've have found this twin pregnancy far more un comfy than a single, and the rate I am growing is unbelievable, but I am sure it is all normal! As for stretch marks, just learn to love them they will fade in time and cover yourself in as much bio oil or cocoa butter as possible!
If you are worried juts go and see your mid wife I'm sure she will put your mind at rest
RM
xxx

Beautiful boy born in January 2007
TTC second since March 2009
Luteal phase defect - 4 rounds clomid
BFP 14.2.2012 - 2 beautiful heartbeats seen EDD 22nd Oct - Its a boy and a girl!