What you are describing sounds like you have decay on the side of your tooth. This would not be related to your recent illness of mononucleosis
. However, the fact that you were in extreme pain at one point may indicate that the decay has infected the nerve
of the tooth. Sometimes the pain will subside once the nerve dies. Which would explain why the pain went away after some time. The only solution if this is the case is to have a root canal
done on the tooth and the decay removed. I recommend you see your dentist
immediately. Tooth decay
is progressive and if you prolong getting this addressed, the tooth at some point may not be restorabe and require extraction.
Hope this helps,