Thanks for the suggestions ladies!

I can rule out dehydration and anaemia. I drink plenty during the day and I had my bloods done the other week (iron levels were very good).

Maybe I'm having the same problem you were Linzy, on my feet all day doing lots of physical work and not sleeping very well because of the shift work! I do have low BP normally, but these episodes are happening when I'm already up and about doing stuff, not the usual presentation for orthostatic hypotension...

Me thinks I might need to go chat to my midwives or a doc!