Sides effects while being weaned off Atenolol

I started taking Atenolol 12 years ago due to a fast heart rate from a panic attack. I was rushed to the ER, had blood work done, EKG, Echocardiogram and all was fine. However, my doctor decided to keep me on it at the time and now here I am 12 years later. My current doctor does not think I need to be on it so we are gradually weaning me off. I was taking 25mg once/day and now I am taking half that dose. It's been 6 days and for the last 4 days, I've been getting headaches in the afternoon and my legs feel achey. Is this a common side effect while being weaned off? If so, why?