Breathless at 23 weeks?

Kayla
Anyone else having this problem? I'm getting breathless just standing for long periods of time (even in the shower). By the time I sit down my heart is pounding :( 
I almost fainted twice yesterday due to the breathless feeling. 
Any tips or tricks to help? It comes and goes but today I have a lot of housework to do, and I can't stand for too long :(