Is it bad if you might be pregnant and a guy keeps nutting in you . Will it mess up the baby?

If you don't know that you are pregnant, you are what, two months or less pregnant

By "nutting" I assume you mean having sex?

By "messing up" I assume you mean hurt.

So by the way you said this, I will assume you are 12 years old or so ?

You need to find out if you are pregnant, and you need to have the doctor see if there are any issues with having sex. Normally there is no issue having sex in the first months of pregnancy, but only a physical exam by your doctor will know for sure

By "nutting" I assume you mean ejaculating?

"Mess up the baby?" How? No, it won't do anything to the baby. Sex is actually healthy in a healthy relationship whether pregnant or not.