Have you discovered what it is by chance?
I just noticed my kitten has a bump an inch up her tail (closer to bum than tip).
It almost feels like a very large tail bone but just the one segment. Shes only four n a half months old and she just saw a vet last week...
But when she walks and flipps...
I didnt know about the nipple thing but my cat had her kittens about six months ago and the one big red flag for me- was when i pick Auggie up- normally i can use both hands to lift her her holding her chest and lower abdomen.
Pregnant Auuggie will NOT allow it if my hand or arm comes into...