Is my pet rat, Cheddar, pregnant?

I just got two 4-week-old rats, Cheddar and Remy, for Christmas and the person at the store thought they were the same . They are currently about six or seven weeks old. A few days ago my mom and I saw them wrestling. Today Remy was nesting while Cheddar slept. Cheddar has been gaining some weight - her belly is bulging out and the hair on her stomach is kind of thin on the sides. Is Cheddar pregnant? Thanks in advance!