Definately sounds like low BP to me Ive always had low BP so prone to blackouts etc if I get dehydrated. My doctor is also monitoring now I'm preg. When I went to get my HCG results at 4w2d it was at 95/65. Id still see your GP simply because passing out when preg could be dangerous to bubs (getting knocked etc). Make sure you never go from lying down to standing up, that still occassionally gets me lol. And just make sure you are really well hydrated as the weather heats up