There are many simple things this could be, but obviously the only way to know for sure is to be seen by a professional, like your dentist.

One of the rules of thumb for problems in the mouth is this; most common problems resolve on their own within two weeks. If it has only been a few days, it isn't time to worry yet. If things continue, then in spite of your nervousness about seeing your doctor, please go. In all disease, and particularly with dangerous conditions such as cancers, those individuals who get the earliest diagnosis while the condition is small, have the best outcomes from treatment. Delaying any treatment only makes things worse. So if you've had something for two weeks and it isn't going away, or you are in any discomfort....please go see someone who can determine exactly what the problem is.