I'm surprised that your OB was mystified. This is actually pretty well documented - a certain percentage of pregnant women get pre-eclampsia or eclampsia (which are mostly high blood pressure with a few other symptoms), and then others don't have blood pressure problems during preganancy, but they do right after the birth.
It is definitely serious and should be medicated as soon as possible. If not, your blood pressure can actually get higher and yes, you would be at risk for a stroke. The medicine brings your blood pressure back to normal quickly.
I had pre-eclampsia and was on blood pressure meds for about six weeks post partum.