Pictures would be helpful - take some shots climbing up the cage bars. Also, in case Jerry is a girl, take some pictures of her standing up as those will show whether she looks pregnant (pregnancies don't show for the first couple of weeks, so if she does, then the litter will be imminent).
The only way to share photos is by uploading them first to a hosting site such as photobucket. Once they are there, you should find an option to share the picture, you need to pick the line of code which starts and ends with image tags (they are IMG encased in square brackets).
There are instructions halfway down this thread
, although photobucket keep changing their options so the code may be in a different place now.
You should be able to confirm which gender Jerry is by comparing with the photos in the article ElizabethW has linked to (the easiest thing to look for is a vagina, a hole half way between urethra and anus).
If she is a girl, then you most likely have a litter on the way - and given that you've had them a month, the 23 day gestation could be up very soon. Rats can become pregnant again immediately after giving birth, so please separate them now just in case (put Sailor into a carrier if necessary).
Then, return to the pet shop you got them from and demand two cages - one for Jerry to give birth in (a low barred cage like a ferplast mary would be ideal, 80x50x37cm), and another for the baby boys to move into when they reach 4.5 weeks of age (a ferplast furet plus would be great, 78x48x71cm). Also ask for bedding and some muesli style food. Don't agree to let them have any of the kittens back to sell though, as they will just be profiting from their mistake, and won't vet the new homes in the way you will want to.