What's considered normal?

I'm almost 28 weeks pregnant and lately I've been very out of breath. I know that's to be expected, but it happens when I'm doing nothing. On Sunday I had an episode of feeling short of breath, dizzy, and light-headed. I had to sit down for about 30 minutes before I felt okay again. These breathlessness are so random and are over with quickly. When I called my doctor they said to go to the ER when that happens--but I know it won't last long so I don't see the point.

So my question is how normal is normal when it comes to being out of breath at this stage?