Need your help! Tank is 2 years old and limping. He normally lays at the foot of the bed and I have a feeling he jumped off the bed in the middle of the night( something he had been doing since her was a pup). He had been limping all the time and now just occasionally limps(like after napping) I felt the back of his hind leg and felt a mass the size of a quarter deep in the muscle tissue...He doesn't wimper with palaption but seems to baby the back leg. Any answers? I have been giving him some benadryl for some of the inflammation...Can dogs tear an ACL or something? Thank you!!!!