My boyfriend is working with an ex of mine. I don't want to tell him, I worry that telling him will distract him in his work.
That's complicated. Do they work closely with each other?
Yeah, they work in the same group and they are becoming good friends.
Sorry that you are in this situation. You should tell your boyfriend about your ex, though. Eventually he's probably going to find out and then he will be very upset that you didn't tell him.
Do you think that I am cheating on him because I won't tell him.
Not it is not cheating but it is a mistake to keep things from your significant other. It will hurt the trust he has on you when he finds out the truth and it will damage the relationship.