Is aspirin dangerous after hitting your head?

Can Be. Aspirin interfers with platelet function, which is required to make a clot. If you take a lot of Aspirin and injure some of the veins along the surface of the brain, you can get a subdural hematoma which is pretty serious. Note a single dose is not usually a problem. Repeated doses where all of the platelets are effected can lead to easy bleeding.
It depends. Aspirin promotes bleeding by ihibiting platelets from sticking together and forming clots. Should one hit the head with a certain force or in a certain way, vessels could be torn loose inside the brain or in the covering of the brain and cause bleeding that could be more severe that if one were not on aspirin. If you have concerns or risks, discuss with your pcp.