Sounds like a muscle knot, it'll go away by itself eventually. Warm compresses help*,
gently stretching, eat a banana a day for the potassium and magnesium. Some people get knots because they're deficient in calcium.
It could be a cyst or lipoma (lipomas are benign) but since it showed up right after overdoing it, I'm guessing it's muscular.
*don't apply heat to areas that are swollen, hot or red. Try cool compresses instead.
PS - I'm a 40 year old chubby woman - there seems to be alot of us here on the healthboards! lol