My brother just had a similar bill for his dog... A mysterious infection that made her leg swell and took multiple rounds of antibiotics and IV/overnight stays to clear up. Then they realized the cause of the infection was a torn tendon on her leg which they had to repair through surgery. Incidentally through all these tests they were doing for the infection they discovered a highly treatable cancer which resulted in further surgery to remove a softball sized tumour. Dog is 8 so she's likely to still have a few years in her if all goes well...