M.I.
well, to pretty much eliminate any discussion, this can happen at any point in pregnancy because the uterus depending on its exact placement and etc etc it can put more pressure on the uterus at any time. at 3 months pregnant the uterus has already grown to the size of a grapefruit.. where just 6 weeks ago, it was only the size of a plum.. thats MORE than double in 6 weeks, not to mention the growth it made in those first 6 weeks. its bigger than people think!
so though i spose it could be twins, i wouldnt get carried away. its probably just normal from pregnancy. women who are pregnant should NOT lie on their backs because of this risk of cutting off blood supply to the uterus.
good luck