What Causes Bruise On Forehead?

I have an unexplained bruise on my forehead. There has been no kind of injury but the bruise, which is about the size of quarter, is very tender. Could this be the symptom of somthing serious? I am 65 years old, weigh about 126 and was diagnosed with sarcoidosis about five years ago. I have impaired lung and throat function as a result. I have been recently treated for kidney stones.