If you do a search on the forum, you'll get a lot of info on this - basically pregnant piggies are extremely pear shaped, usually - ie, they are 'nomal' sized on the head end, then get very round on the back end. From above they look like they have large lumps on either side near their back legs.
If you are feeling movement, it is most likely babies. You feel movement starting around 3 weeks or so before birth.
I'd recommend taking to a cavy savvy vet though, just to make sure. Other things can cause a distended belly that isn't pregnancy.
Good luck. If you can post pictures, especially a view from top looking straight down, that might help. Many of us here got the baby bonus when we got females that were already pregnant, either from a petstore before we knew better, or from a rehome/rescue where the sow had been in with boars at some point.