What's causing my fainting and vomiting?

[Takeabreathandcarryon]

I'm really struggling with what I'm guessing is postural hypotension.
When sat down I can check my blood pressure and it's fine (usually about 110/70).  Then I stand up, immediately feel horrifically dizzy, often faint, and frequently vomit when I faint.
I feel really drained.  And however slowly I try to stand up, I'm still getting dizzy.
I'm generally fairly healthy although I do have a dental infection at the moment and am taking a course of flucloxacillin.
I don't know what my blood pressure is doing when I stand but assume it's dropping.  I can't measure it at that point because I'm too dizzy to do the reading (or else I'm laid on the floor in a puddle of vomit!)  Sometimes when I stand I have something akin to a seizure - my arms and legs spasm and shake uncontrollably.
This is making life pretty tough and I don't know what's causing it or what I can do about it.

[Dandeliodeer]

Do you have gas heating in your property?
When was the last time you did the service.
Dizziness with vomit could be an indication of carbon monoxide poisoning.
It is extremely serious and better to stop it early as it can ruin your health if it becomes  chronic .

[Thepoopsnoop]

Have you been checked for anemia?

[Takeabreathandcarryon]

Not been checked.  I eat a lot of green stuff so would be surprised if I'm lacking iron but it's certainly something I'll look into.