I hit my head fri on a desk bending over, very tired, I was sleep deprived also had nausea 3 night later but doctor said it was unrelated? Concussion?

Unlikely. Concussion is an acute event immediately following head injury. There is alteration in neurological functioning that can be defined by specific testing immediately after the event. Delayed symptoms may be due to stress or viral illness or medication effects. The term concussion is not as important as the management of your current stress condition.
Hard to tell. The degree of trauma you describe is usually not associated with concussions. However, if you've had a history of concussions in the past, minor trauma such as this may suffice. Symptoms to watch for include, headache, nausea, light sensitivity, drowsiness, and difficulty concentrating, among other things.