What excuses can you tell your football coaches if you think you got a concussion but it was not while playing football?

Tell the truth. You could simply say:

You: "I think I have a concussion."

Coach: "Why do you think that?"

You: "My head hurts."

Coach: "Did you see your doctor?"

You: "No."

Coach: "I suggest you see your doctor and have him write you an excuse if a head concussion is the diagnosis."

But, if there's some OTHER reason you just don't want to play football, maybe it's time to address that. Think about whether you really want to play or if you want to stop playing. Maybe you really just want a week off instead. Whatever your real feelings, just think about what you really want, and be honest. Remember, coaches put in a lot of time and effort and deserve that players be honest with them.