What does it mean if you have both fast heart rate and anaemia?

Possibly nothing: There are many causes of anemia, and if the anemia is severe enough, it may be associated with a fast heart rate as a normal compensatory mechanism to maintain adequate cardiac output and provide sufficient oxygen delivery to tissues. Once the anemia is treated or resolves, the fast heart rate usually normalizes as well. A doctor's visit may be necessary to determine the cause of the anemia.