My daughter is 5. She recently had a very swollen and painful foot for no reason. No know injury. Her esr was 95. what could cause this?

Inflammation. Swelling and elevated esr could signal an infection either in the joint or bone, or an immune type arthrtitis like rheumatoid arthritis, lyme disease, serum sickness like reaction. History as to recent drug intake, other symptoms like skin rash or sore throat, fever or swollen lymph nodes may give clues, in addition to other labs or imaging studies.