Fainting in 3rd trimester??

I’m seeing my doctor today but just wondering if anyone else has had this experience.

I’m 28 weeks but for the last few weeks, I’ve experienced feeling out of breath and fatigue in my legs sometimes when I walk or go up stairs. It’s not all the time but it happens quite often. I can still work out just fine and I’ve stayed active, so it’s not from excessive weight gain or lack of activity.

Well, today, I tried to “push through” the feeling and keep walking (I was at work so didn’t have the option of just sitting down). Well I almost fainted and had to sit on the floor. I was sweating and feeling nauseated. I managed to avoid totally passing out but it was close. This has never happened before! Any experience with this sort of thing?? (For reference, I do not have GD - got tested at 23 weeks)