Dizziness and nausea

Sheena
I am 28 weeks and 2 days pregnant. Twice this week, I have experienced a sudden onset of being faint and dizzy which was then accompanied by extreme nausea. I had not been doing anything extreme and was also not in a climate that was hot. I had simply just been taking a walk outside and had been roaming my yard for less than 10 minutes. Is this normal? Afterwards, I've had to lay down for about an hour to feel decent again.