New optometrist changed myopia prescription to astigmatism prescription and have had a migraine ever since. my old glasses don't do that. what happened?

Takes time to adjust: To the new RX, but this will never happen if you always go back to your old glasses. It would be worth having the glasses rechecked.

Go back to optom.: Glasses are sometimes made incorrectly. Other times the refraction (measurements) are done incorrectly. You shouldn't be having headaches from a prescription which is supposedly 'better' than the old one. Go back to the optometrist.