What is wrong if u r a tenage girl and ur foot is swollen and everything below the knee hurts?

its been like that 4 four days and i had a cold last week so i didnt do anything physically straining and i just woke up one day and it hurt and the drs dnt no wats wrong cept i got an ultrasound and its not blood clotting and i got a blood test and it was okay and they dnt think its infected and they'r really concerned, any suggestions?