Originally Posted by
adacas
see a doctor
Yes--especially if it's only started recently. But even if it's something that you've been experiencing for decades, ask your doctor about it.
People with mitro-valve prolapse (heart murmur) sometimes experience headaches when they exert themselves over a long period--especially in hot weather. MVP is not a serious problem, in most cases, though it's something a person should know about, if they have it. I don't mean to suggest that you have MVP; I only give this as an example of something important, and yet usually harmless, that a doctor could tell you about.
Or it could be that you're not eating right before your rides.
And of course, there's a small possibility something new and bad is wrong.
See your doctor, if you feel worried about it (which you are, or you wouldn't have posted).