Vitamin B100 and magnesium, every day, are very helpful to ease or eliminate that kind of headache.
Sorry to hear about the hysterectomy. You may still have enough hormones in your body explaining the mild symptoms (hot flashes). So taking the HRT may have given you too much estrogen thereby causing the headaches.
I also had a hysterectomy and did not start HRT until around 6 weeks post-op even though I was not having many symptoms (the main one being inability to sleep). However, the HRT was not adequate for me because I ended up being "slammed" with symptoms (with hot flashes being the least of them). I was in an almost non-functional state for too long because my gyn surgeon abandoned me when I called his office and then I was told by my next doctor that my estrogen was at a therapeutic level.
Even if you get by with few symptoms, medical studies show that estrogen deficiency caused by ovary removal has many adverse health effects, some silent and/or longer-term and others more apparent and immediate. You can find the studies in PubMed by searching for something like "bilateral oophorectomy long-term health."
The ovaries of women with all their parts produce hormones (mostly testosterone which can convert into estrogen) into their 80's for optimal health and well-being.
Hope this is helpful!