Don't worry, it will feel much better when it quits hurting!!
Several years ago I tried a discount abrasives/el cheapo 50 grit belt and had a similar experience.
While flat grinding a blade, the (new) belt snapped in two with out any warning. It grabbed the blade and slammed it into my chest directly over my heart and dropped me to my knees. It hit me edge on (not point first) and cut me to the bone.
Had it hit point first it would have killed me.
Sometimes we can get so used to our tools that we forget that ALL power tools can be very dangerous. Complacency can kill.
I learned my lesson and will not be using any more cheap belts and I ALWAYS inspect the belts before I turn my grinder on. If it looks even a little suspect I throw it out.
In this case the belt was new so I had no reason to suspect any potential problem. Just a cheap belt that came apart at the splice.
Now ALL belts get a close look before I run them and as Rusty said, turn them on from the side and let them get up to speed before working with them.
-Mike-