Maybe you had a misidentified mouse that was actually a male in your feeder bin, but sometime after he impregnated the mouse in question you fed him off?

Although, if that were the case, I'd expect you to start seeing quite a few more pregnant mice.

There are at least 2 other possibilities. If a mouse pigs out on something, they can get a bulging belly that looks very much like the pregnancy. This will only last until the food in question has been digested. Also, mice can get obese and this can resemble pregnancy at times as well.