cat health problem....need advice!

My cat Jess had her first litter of kittens, 9 weeks ago.
She had 2 kittens which have both gone to new homes. Since they have left i've noticed she has a lump near her back teat, it's as if it is still full of milk, even though she hasn't been feeding for the last 3 weeks, and her other teats have all gone down. she doesn't complain and is her normal self ( eating , playing etc.)
but i'm worried, does anyone have any advice, should I take her to the vet, or is this normal?
please help
thanks