NickyD,
Could be Orthostatic Hypotension like Cajungrl said ( I had that more in my early 20's long before I was diagnosed, I used to have drops in my BP for no reason I normally run 90/60 and would drop to around 76/48) the doctors that I told about it didn't seem too concerned, maybe because it didn't happen right in front of them and they didn't believe me. I worked in a hospital at the time and it used to freak out the nurses when they would take my BP. It could also be an inner ear infection, or dehydration.
Take Care, KO