Could hamsters be pregnant for 2 months?

I bought a hamster in middle of March this year.. and last week i found that she had a litter of pups. it is now June 2nd.. I've researched online and found that the gestation period of hamsters ranges from 15-30 days depending on the species. I don't have any other hamsters in the house.. and no one even knew that she was pregnant? I guess the pet store sold me a pregnant hamster w/o realizing it.. but how could she have been pregnant for such a long time, practically 2 months??? everyone is joking around saying that a mouse went into her cage because we cannot solve this mystery.. WEIRD.