Blood pressure

Over the last couple of weeks I've experienced several temporary bouts of dizziness. It seems to occur if I'm bending down and then stand up too quickly. I checked my blood pressure the other day and it was only 99 over 51. This is classified as hypotension - i.e. too low. It's not dangerously low - my kit says to see a GP if the low blood pressure is accompanied by fainting. So I'm OK, just a bit curious as to why it has apparently fallen. It's never been high - even when I was 20 stone my blood pressure was always normal. I noticed when I first got the band that my blood pressure was low but this was down to dehydration. It can't be that now because I make sure I drink at least 2.5 litres per day. The only thing that's changed in the last two weeks is that I've got 'back on the wagon' and begun to lose weight again. Maybe it's just my body saying "Whoa! What's happening?!"