I can't stop taking them though because I have no thyroid and a non working pancreas. And I've been on them for years.
In that vein, I, too, take daily thyroid medication, and at one point I was having 3-day migraines every week. Then I discovered that the BRAND of thyroid meds I was taking had cornstarch as an inactive binding ingredient (starches are a major trigger of lots of my symptoms, including migraines, fatigue and Sjogrens Syndrome). I carefully researched all available brands of L-thyroxine until I found one with no starches and no talc as binders (sadly, I couldn't find one I could afford that didn't have food colorings, but I tried).
Once I switched to a no-starch/no-talc formula, the majority of my migraines went away (I now only get one-day migraines, and on average they're twice a month, not every week)... a MAJOR improvement in my life.
Not to say food and/or environmental triggers are causing what you're dealing with, but they may be complicating them, plus eliminating common triggers is one of the few things under your control right now.
Check out my post & poll "What are YOUR triggers?" from last month for some ideas. Since this problem is occurring when you go to bed and you need double doses of antihistamines to sleep, I'd also recommend dustmite-proofing your bedroom (use dustmite/bedbug covers for pillows & mattresses; thoroughly wash any curtains in fragrance-free detergent and stop using terrycloth items & thick blankets - mites love them. If this helps you sleep off some of your buggy episodes and migraines, then get rid of any carpeting in your bedroom (full of mites, guaranteed).
Sometimes getting things under control requires thinking well outside the doctor+drug box!