poopy butt, yellow mass underneath

[etakyram]

hello all! i'm new to posting here, but i've been reading your posts ever since i got my peeps in march!
my dopiest chicken always tends to poop on her butt feathers. her poops are always normal. but it finally clumped up so much that today i gave her a bath to get it off. it wasn't easy, but i succeeded for the most part, and i plan to trim her feathers down tomorrow to prevent it from happening again.
while i was down there, it seemed like she had a quarter-sized, yellow, round, swollen mass on her butt. it's located well below her vent, and to the left, midway between the vent and her underside/legs. it's not balding, but hidden by her butt fluff. i couldn't get an excellent look at it or a picture because she was not happy after her bath!
she is otherwise acting completely normal, and she laid an egg this morning. has anyone seen anything like this? i appreciate all your experience and input!!

 

[Eggcessive]

It's possible that it could be a tumor or an abscess, but a picture would help. Poop can also turn to cement sometimes, so it may need more cleaning.