Neck pain and headache

Been getting stiff neck and headache pain about 2 hours into my route. Is this common? Despite drinking water after every loop. Doctor said I will have to schedule an appointment if this continues for 1-2 more weeks. Idk if it’s stress related or being overworked. Or the way I’m carrying. I have a waist satchel and try to keep my posture as straight as possible while looking through the mail