Hi
Welcome to the MedHelp forum!
Post exercise nausea or exercise induced nausea is usually due to over exercising, or abruptly stopping exercise, or over hydration. Drinking too much water before, during or after exercise could also be the cause. Eat some salty snacks and see if it helps. Exercising on empty stomach which causes blood glucose to dip with exercise can also be the cause. One should exercise after 2 hours of a light meal.
Exercise can be a trigger of migraine which will cause headache and nausea. Exercise can also trigger electrolyte imbalance, dehydration, spinal problems etc all of which can cause headache and nausea. Your blood pressure too can shoot up with exercise and cause these symptoms.
I think you should discuss these possibilities with your doctor. It is difficult to comment beyond this without examining. A comprehensive investigation is required keeping all the points in mind. Take care!
Migraines, Occipital neuralgia
I am guessing it starts or is the worst right in the back of your head where the neck and head meet?