Sounds like a hematoma, if it is hot to touch I would be concerned about infection. Could very likely be caused by her pinching her chest between her hooves when she was trying to lie down or get up.
Check for any puncture wounds near the lump which would be your entry site for bacteria. Since it is sore and hot to touch I think you should have a vet come and look at it. Hematomas are usually not serious unless they become infected. If they do become infected there is a risk the infection can become systemic as there is access to the circulatory system from the vessel leakage, this is a serious complication.
All horses deserve, at least once in their lives, to be loved by a little girl.
Last edited by sarahver; 05-26-2010 at 10:05 AM.