Good news! The pathology report came back, and Lido had an encapsulated hematoma, which is what my vet thought it was. Apparently, it was a VERY nasty carpenter bee sting, which Lido's body reacted severely to - hence the swelling and bloody fluid when aspirated. The prednisone reduced the fluid, but left a hard knot, which was the dried up hematoma. I'm glad I had it removed. That lump would have been permanent, and it was in the shoulder muscle - very prominent and unsightly (about the size of an unshelled pecan). I'm so relieved!
I had a boxer seven years ago, who had a lump behind his ear, and my then vet said it was probably nothing (boxers are notorious for getting cancer, btw...). Two months later, he had to be put down due to unbelievably fast progressing cancer. I'm understandably gun shy now, because of this. Better safe than sorry! I'm glad it wasn't a limpoma, as those frequently grow back.
Thank you everyone for your well wishes and good thoughts!!