C: have I mis-sexed my gerbils?

[Susan Batey]

Help! I just joined this list today because I have a unique gerbil question and need an immediate answer. I acquired four black gerbils when I quit teaching Pre-K. Two were ultimately killed presumeably by the other two. We now have 2 female gerbils who have lived with each other exclusively for over a year and a half. About 4 weeks ago, one of the gerbils escaped and was missing for over 2 weeks. One of the cats ran into the house unnoticed until she pounced into a pile of clutter under the chair--you guessed it--she came out with friend gerbil in her mouth, dropping her several times until said gerbil escaped again and  hid in an overturned bucket. I threw out the cat and grabbed the gerbil--suffering a severe bite to the hand. I then reunited the two girl gerbils and that was that. Until..... the once-escaped gerbil started looking like Jabba the Hut--this gerbil has gained a lot of weight since her return two weeks ago. Anyone have any idea what I could be dealing with? Do gerbils ever crossbreed with other rodents? My husband is concerned the gerbil may have internal injuries from the cat but that was 2 weeks ago... Please answer ASAP.