What Causes Elevated BP While On Medication For Hyperthyroidism?

I never had high blood pressure it was always 110/70 to 120/70 why now is my blood pressure much higher, I had cervical neck surgery almost two months ago and found out I have many nodules and a hyperactive thyroid....just started meds for the thyroid! Its goes up an down for the past two day between 158/99 to128/90 and any where in between every time I take it, what can be causing it? Thanks