Interesting...

So I needed to clean out my feeder bin today, little background, I feed off all my males before 4 weeks to my ferrets and only keep females for feeders.

One of my feeder females is pregnant, very much so. I took every mouse out of the tank and sexed them twice, all females, including a couple in heat so I watched the tank and them for a while and didn't see any chasing/humping going on. None of the others look remotely pregnant. I haven't fed many off lately because my snake was fasting but I know the last couple feeders were female for sure because they were retired breeders.

The tank is glass with a mesh lid so I don't believe a loose male could be the culprit either. She's out of my last litter and is 2 months old which puts an early blooming sibling out of the running. I take the dad out before they are even born so that's out. I'm waiting to see if maybe she's not pregnant but ill, by the looks of her she should drop within a few days and I put her in her own tank to keep an eye on. I guess with any luck I will get an extra litter out of this but still very confused on how this occured.

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.

Well she's by herself now and I'm wondering if she's ill, I cleaned cages today and this mouse sat in the corner breathing heavy the whole time, when holding her she acts normal but we shall see. They have been fed a weird diet lately because I was waiting on food to come in, it got in last night so it's possible she pigged out. I'll know in a few days, either she'll have babies or she'll die. I'm going to keep an eye on the bin and make sure none of the others show up pregnant, I would think if it were a missexed male that the older females would be PG as well. Who knows.