Bullmastiffs

Hi,
I recently discovered while bathing my bully that she had a lump under her tail, pretty big, not noticeable unless the tail was being inspected, but its pretty large, I haven't had her long and she is over 4 yrs old, I asked her previous owner about it but he said he never noticed it, which I don't doubt. Has anyone had this issue before? It doesn't bother her I can touch and squeeze it and it doesn't face her but I'm concern, other than that she is happy healthy, active and in excellent shape. If anyone has come across this issue your impute would be greatly appreciated.
Thank you in advance