I recieved a letter from my doctor should I be worried?

I received a letter saying could I please book an appointment with my doctor at my earliest convenience. I've never received on these letters before and I was wondering if it is something bad? I'm really scared I'm only 25 and i've had a check up recently and they told me I was ok. So what's with the weird letter? Has anyone received one of these before?? Should I be worried?