Dehydration generally lowers blood pressure.
Stress will raise blood pressure (inner ear issues will cause that too, as body is getting mixed signals).
At this point I'm 99% sure you have inner-ear issue as Faygo stated.
Talk to doctor at next checkup see if they will give you prescription for Menieres Disease. You can take at the onset to ease the issue.
Also some blood pressure medications do cause the symptoms of Menieres Disease. If have changed anything, check into side-effects.