I've had this too. I almost passed out on my commute on Monday morning, then twice again on Tuesday evening. My husband was so worried, he ended up taking me to A&E. The doctor took my obs etc. and said everything seemed fine.
I've spoken to my fertility clinic and they said dizziness is quite common but I'm going to get them to take my bloods tomorrow, just to be on the safe side. I'm literally getting it even when lifting my head up to move position in bed x