I cheated on my boyfriend
What would you do if he tried to retaliate in a harmful way?
I'd say he is probably joking
Joking or not, you admitted to cheating on him. That is not something that is okay for anyone to do.
I know but I'm in a good mood today so I thought I would tell him.
I'm sorry this is happening to you, but it is wrong to cheat on your significant other. If you care about him you should show him more respect. You should try and be open and honest with him now.