help plz..something inner body..??..

something like a ball in my rabbit stomach or udder is inner into the rabbit not outer cant see but just feel when palpating
the rabbit have 4 months dont know if male or female yet, i wanted to know by palpating the nipples to know if female!! (i know is silly way, )) but i found the small ball inner, is moving with my fingers and is flexible as it swim in some watery space ...
the udder outside is perfect, no any problem with health and is good active rabbit,
hope anyone suggest what this ....

Is it a small round ball on the right side of the tummy? It's probably just poops forming in the cecum. My bunny has it too.