Honestly, I think blood pressure drops due to dilated vessels in hot weather. I wrote the other day about Baroreceptors, they're tissue that measures the stretching of the vessels in key areas. One article said if your blood pressure rises too much, your heart slows down trying to balance the pressure, conversely, if it drops, your heart speeds up to try to maintain the pressure.
In the past, I've been coming down with a cold, early stages, getting a lot more ectopics than usual.