Your description strongly suggests that it was a hematoma, or "blood blister". These areas of internal bleeding under the mucous membrane tissue surface are usually the result of minor trauma, such as an inadvertent cheek or lip bite, and cause a small collection of blood to pool under the tissue surface in a small blister. Typically, these blisters rupture, leaving a small ulcerated surface that heals within a few days without the need for any intervention. If this blood blister was located on the cheek or lip at approximately the level where the upper and lower teeth meet, this would be further corroboration of its identity.
Usually, these events are the result of simple accident. In other cases, you may be able to identify a sharp or malposed tooth, or a denture
clasp, or some other area in the mouth that due to its surface contour or position may be likely to cause additional injury in the future, and in these cases it would be appropriate to have your dentist
smooth out or eliminate the offending sharp surface to diminish the risk of it happening again.
In some cases, blood blisters can recur frequently without any apparent physical cause, and in these cases it would be prudent for a formal medical diagnosis to rule out any blood clotting disorder or connective tissue disease which may predispose to bleeding.
Hope this helps...