Mine came on suddenly too I had been feeling 'off' all day and it was mega busy, it wasn't from getting up from a sitting position or turning my head, I was half way through helping an elderly couple buy a mower. In the end I ended up crouching beside the mowers still talking to them but just in case I was sick I got down low lol. Luckily they chose one quickly and left the aisle and I made it to the staff room in time, where I lay drinking a can of soft drink and eating a chocolate bar as I thought I was having a sugar hypo. Didn't help I ended up going home and felt nauseas for days before I saw the doc and she told me it was low blood pressure combined with dehydration for me though.

My blood pressure has gone back up now (as I hear it often does at this time, but we are all different I guess) and I'm not feeling as bad, though I can't spend more than an hour on my feet in a shopping centre anymore, or I get sick. Making it very hard for me to buy baby clothes and things, I have to send DH out and he knows nothing (it's his first, my second) about babies! hehe

I hope you sort it out soon