I had this and it turned out that I had a subglotic tracheal stenosis that required surgery. If you are having trouble breathing, get an appointment with your GP ASAP. First they tested me for asthma, then I saw an ENT, then finally they did a bronchoscopy and found the large growth of scar tissue that was slowing closing my trachea. I guess with the hormones in pregnancy, the scar tissue grew a lot.
GL