What Causes Pain And Redness On The Foot Post Antibiotic Treatment For Strep?

6 year old female . One day after completing antibiotic for strep, developed pain on top of one foot with redness and swelling. Pain and swelling resolved in 24 hours, however 12 hrs later complained of pain behind both knees. Redness behind each knee. Strep infection detected at time of physical. No throat problems evident but slight swelling of node on right side of neck. Throat swap done as a precaution but considered able to have flu shot that day. 10 days of antibiotic given