You were drunk and fought a semi pro boxer in a bar and took a heavy blow to the back of the head two days ago the pain feels like brusing but theres no bruise or mark at all should you be worried?

Pain means that there's some type of bodily damage, and it doesn't have to leave a surface mark or bruise, though that's usually indicative of more serious damage.

Without further testing (X-ray, MRI or CT) you can only speculate. But I will tell you that if the person you fought was a trained fighter (pro or not makes no difference in the eyes of the law) and you were injured, then you have a legal case, depending on who started the altercation. Of course if it was you, then keep your mouth shut - then it becomes a case of assault and self-defense.

This is something I used to deal with (and still do) when I was still an active Black Belt. It was used against me more than once though I never did anything to anyone.

You should at least get pictures taken to ensure theres no cervical spinal damage either. Such damage won't necessarily be readily apparent, but it can lead to a lot of pain later.