Sam,

Don't try to push exercise to build more tolerance until your other symptoms have resolved. Then, slowly step up your effort level always staying below your head ache threshold. Use one week or two week steps. Building up tolerance is not a daily effort. When you can take a 20 minute walk everyday for a week or two without causing a head ache, then you can increase it to 25 or 30 minutes for the next two weeks of head ache free repetitions.

Otherwise, you risk regressing.