What happens if you fall and u bruise and you are on a blood thinner ?

The same as when you are not. Bruises represent the effect of blood leaking out of capillaries close enough to the skin surface that you can see the stain they make. Those cells break down and cause the color change. The material gradually changes color and is absorbed back into the circulation slowly. Other than being a noticeable event it has little significance.

Bleeding may be prolonged and result in deep hematomas or collections of blood that are not visible. Tight follow up is always recommended for 2 reasons: 1. To make sure bleed has stabilized/stopped and 2. To look for ways of preventing possible future falls/ head trauma that may be life threatening.