My Son Was Diagonosed With Throat Ulcers A Month Ago

My son was diagonosed with throat ulcers a month ago now that's clearing up and now his ankles and knees are swelling and know one seems to know whats wrong now swelling is getting worse and he can barley walk been to docor and the er they saying nothing they did blood work told me that came back ok so hes ok and obvisely not ok.