“Sorry, I got sidetracked.”

Someone asked you a question earlier in the day at work. You were supposed to find the answer, but you forgot because you were doing other things. The person calls you to ask again, and you say:

Sorry, I got sidetracked.

Sorry.

This is a way to apologize for a small mistake you made. When you make a larger mistake and want to apologize more strongly, you can say "I'm sorry for ___ing":

I'm sorry for making you wait so long.

But "I'm sorry for getting sidetracked" would sound strange, because getting sidetracked isn't what you're really sorry about. You're sorry for not answering the question quickly. So a more serious apology would be:

I'm sorry for taking so long.

get sidetracked

When you "get sidetracked" it means that you were trying to do one thing, but then started doing something else instead. You can say that you "got sidetracked" by something more important than the original action, or by something that's not important.