There are definitely some changes. Depending on how far past menopause, and what level of estrogen is still in your body at the time you start tamoxifen can make a difference. For me, at 67 with fairly low estrogen levels and 17 years after menopause, I could feel my body making adjustments. The first week I had some warm flashes and moodiness and feeling “off”. That increased at week 3 - that’s about the time the full affect of the tamoxifen is in your body because of the 7-10 day half life of tamoxifen. The drug accumulates every day until it evens off. I pretty much quit having regular warm flashes and the moodiness evened out. At 3 months I felt more changes - I think by this time all the body’s organs and functioning has adjusted to the tamoxifen, and it affects parts differently.
I’d suggest waiting to see how it goes, manage the things that you can (headache), give yourself some grace to deal with the mood changes, and see how your body adjusts. Unless of course there is a really bad issue, then the doctor needs to know. Our bodies are all different, it’s hard to know exactly what side effects you’ll have.