Typically headaches that are related to more systemic causes, (dehydration, low blood sugar etc), are more global in nature. That is, they tend to be more evenly distributed over the head. They may be more towards the front or back but they are usually pretty even side to side.

Your migraines are atypical in that you don't get an aura etc and that it is more to one side. This makes me wonder if your very nasty headaches are related to changes in blood flow and neurological activity as migraines typically are. Not that it really matters. It wouldn't change any of the suggestions I have.

So what am I thinking so far. This is where things get interesting. From a PT trained in a Western University point of view you could be having a headache as a result of nerve irritation at the upper segments of the neck related to your cycling position. It's a nice diagnosis that accounts for the location and quality of the headache. Unfortunately, it does not explain why it's worse in the heat.

Get ready, cause we're about to get a little weird. So I switched out my brain for my Traditional Chinese Medicine (TCM) brain and I have to say that I think you are suffering from blockage of qi in the gallbadder meridian, with increased liver yang. This explains location, temperature dependency and the fact that it is worse with activity. This diagnosis is also often related to migraines, pms and menstral cramps.

If I were you, I'd go see a TCM doctor who practices acupunture and can prescribe herbals.

I'm going to try to find a link that helps locate TCM practitioners and post it.

I hope you're feeling better.