
Agree with white lilly that I wouldn't be worrying (unless it was hard, inflamed and swollen.) I think that you are more likely to panic if you haven't seen it before.
One of my girls last year had a really huge swollen back teat. Mum had more milk than 3 pups could cope with. She had got into the habit of lying on the one side, and one teat kept getting missed, because it was harder to access for the pups. I turned mum, and tried putting pups on the swollen one purposely. It helped swelling once they had started to 'empty' it.
As they got bigger, and their milk needs were also bigger, teats calmed down soon enough.