Don't worry, it's quite normal to some pregnant mom. Coz 黏膜變薄, it's also affected by hormones. For me, I have different experience which is due to pregnant hormones as well. On the 4th month of pregnancy, I suddently got serve nose allergy for a few days, because the hormones affects my circulatory system, and the blood vessels inside the nose expand which make me feel like 'having a cold' and cannot breath.....my doctor and lots of books said it's normal. Sometimes, the situation will continue, or disappear suddently, and definitely disappear after giving birth. Lucky, it only happens for few days.
Just like some mom got bleeding problems with their gum (the teeth).