Have antihistamines been shown to help with chronic headaches? It sounds like they would if the headaches were due to dilating blood vessels in the brain. Would they only work if the dilation was due to histamine or would they work no matter what the cause of blood vessel dilation was (assuming it is blood vessel dialation that is causing the problem in the first place).
I only have anecdotal evidence, and it only applies during my allergy “season”. However, there is at least one prescription med that is used for migraines which works on this principle; I see a big poster on my pain doctor’s examining rooms’ walls. Nope, don’t recall the name of it; sorry.
Have you ever tried applying cold (i.e., ice bag or small frozen gel pack/“blue ice”) to the back of the neck? It will also cause constriction of the blood vessels to the head. It works well for me, and for a number of other sinus/migraine sufferers I’ve known. OTOH, there are some for which it does nothing.
Another possibility: Do you perchance have TMJ? The links I have are old, so you’d do better, I think to google for migraine + TMJ. Some people have had amazing results from getting their TMJ treated.
Anecdotal: I self medicate my occasional migraine with OTC combination of: aspirin, caffeine, and antihistamine (chlorpheniramine maleate).
Has been working most of the time.
Peace.
I forgot to say, before, that you should always wrap an ice bag or gel pak in something before applying it to your tender, sensitive, etc., skin. 
Or put something in between. You are, after all, going to lie on it, which means that the maximum amount of your neck is in contact with the cold. <brrrrrrr> I bought a pack of those “guest towels” years ago. They’re just the right size to fold around a gel pak, and they’re pristine white terrycloth. Just enough insulation.
I have a special cervical pillow with sleeves attached on one side to hold gelpaks. AAMOF, it came with one. Pillow is filled with poly-type filling, though it must be special high-quality, as it’s more than 20 years old, and is still fluffy. It’s horribly stained, but nobody can see that when it’s in the pillowcase. 
I paid >$40 plus shipping to a catalog company, and it’s still good, except for appearance. I’d say I’ve gotten my money’s worth out of it, wouldn’t you? 
The brand, BTW, is Synektics. AFAIK, you can’t buy it in stores - only online or from some chiropractors. And apparently they don’t make the same model (with gel pak) anymore. 
It’s gonna be a pain, sewing the sleeves on by hand, when I finally hafta replace it.
I think I suffer from cluster headaches. I would call the school doctor but its the weekend and hes closed right now.
I was looking info up on cluster headaches and the symptoms seem to match mine. it also said anti-histamines could help so I started taking those on saturday. I still got the headaches but their intensity was alot less after I started using anti-histamines. Now i’m sleeping 10 hours a night due to the antihistamines, which sucks but not as much as the headaches do.