Please take her to the vets if you are concerned. While you are there, find out how soon after birth they will spay her and remember that she might get pregnant again very soon after birth if you let her out. Keep her in until she is spayed.
How far along in the pregnancy is she? If she is close, the teat is probably filling up with milk. The lower ones seem to fill first. Is it painful to touch? Do you see any redness or unusual discharge? (green, yellow, bloody)
This could be normal if a queen is in late pregnancy and has perhaps been grooming more on that area. It's not unusual for the back teats to be more prominent since they usually are the ones that have more milk and as Levi says, seem to fill first.
. It's normal for queens to lick the hair away from around the nipples when they are soon going to have kittens and it may be she is just doing it more in the back , especially on one side. and that in turn may lead to it filling/swelling up more. .
But also do pay attention to Levi's questions about whether it seems to cause her pain , and whether there is some unusual discharge. If so, discuss this with your vet. If she DOES have an infection, there is a good reason to want to find out because the kittens should not nurse from the infected teat.
Thanks everyone. She won't let me touch her very often so I could never tell if it caused pain. But the swelling went down so I guess she just groomed it a little long or something
About how long after she does that will the kittens be born??