You know, with being on bed rest, the muscles/tendons/ligaments and such that hold up your preggo belly get a bit of a rest, too. Since they're not working so hard tend to lose a little of their strength and elasticity. Soooo, it's possible you're experiencing those bits straining as they take on more load than they're used to. Kind of like round ligament pain in early pregnancy.
In fact, if it's on the side, it might just be round ligament pain....
Just a theory.
Parenting two active, wonderful boys
This is your world. Shape it or someone else will. -Gary Lew