Hello, I am Dr. Selmani and I read your question carefully The plantar fasciitis has other signs different with what your son presents. Usually plantar fasciitis pain on the heal is in the mornings , when you do the first steps and you feel like walking on nails , then after some minutes it warms up and the pain goes away. Females have more plantar fasciitis then males and overweight and sports activities on concrete are related with plantar fasciitis. Your son s pain looks different from plantar fasciitis. I would like to know what is the age of your son and what antibiotic he is taking. If he is taking Ciproflaxacin ( or other drugs from this family) and is young age below 12 years old, the antibiotic may cause Achilles tendon rupture and pain behind the heel. Also other information that I do not know is , how much lasted the pain , seconds , minutes ?. Was the pain related with standing position in weight bearing or even in sitting position. Does he have fever. All this info would help to distinguish if it is a infection or inflammation. I think the cellulitis has left still some sort of inflammation around the foot and ankle that needs to be evaluated from the orthopedic specialist. I wish quick recovery.