K.R.
eep! just read the other responses and sure hope you have already gotten a medical evaluation!
in a less-scary-for-now (but definitely more obnoxious for the long term) response, it could be a migraine (ongoing headache + flashy lights, two classic symptoms) ... but if you've never had one before, probably the other ladies are more on-target.
the least scary answer I can think of, is that perhaps you need a chiropractic adjustment (the pressure can be caused not just by blood pressure/systemic changes, but by local-source physical pressures of bones on nerves and things like that).