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Do you remember injuring it in any way in the last day or two? Lay on it differently?
Is it feeling better in the 24 hours since you placed the question?
Since I haven't heard from you I will propose some possibilities.
You could be having a muscle spasm. It could also be tendinitis, an inflammation in the tendons that attach muscle to the bone. The best treatment is ice applied in a towel or washcloth for 15-20 mintues several times a day.
If it isn't better by the first of the week you should see your doctor for xrays and/or an MRI.