You'll have to see a doctor to find out what this is about. A number of things could cause it; a muscle strain, something called piriformis syndrome or pseudo sciatica-gluteus minimus - which involves tears or strains of the muscles that run from the pelvis to the hip, or more serious problems such as osteomyelitis.
You might try rest, ice and anti-inflammatories to relieve a possible muscle strain/tear. Most muscle strains clear up in a couple of weeks; the general rule is that if its not gone by then its time to see a doctor. Good luck!
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