Shortness of breath while laying down

For the last couple of days I've really noticed that it's hard for me to breath as soon as I lay down or sit down, eventually I'm able to catch back up with it. But it's like right as soon as I do that. I'm 20 weeks and I'm not quite sure what's causing this but it's a scary feeling/symptom. My doctor at my last appointment a couple of days ago asked if I was having shortness of breath and she didn't say anything about it, just kinda acted like it was normal??