Given the history, it might just be a "stump pyometra" which is a persistent infection at the place where the uterus was tied off. This sits just above the bladder neck and can look like a mass. It can result in smelly vaginal discharge. If this is the case, they will just remove the stump (it is a abscess) and put the dog on antibiotics. It would be uncommon for a 3 year old dog to have a bladder tumor, especially a beagle. Try not to worry.