You may have an infection or it may be a keloid scar. Here's a link on how to take care of a lip piercing that would also apply to a Monroe piercing.
What Happens When You Have A Knot Around Your Monroe Piercing/inside Your Mouth?
I have my monroe pierced and it's just swollen. If you just got it then it will most definatly happen, but if you have had it for awhile you're probably just playing with it too much. Go buy some bactine from Walmart, or cvs.