Because the growing foetus was pressing upon your vital organs
when you lay in a horizontal position. Your stomach was being
compressed, making you feel nauseous, and this in turn would have
pressed against your lungs, restricting your breathing.
These symptoms are not uncommon during late pregnancy, which is
why many pregnant women sleep sitting half-upright in their latter
stages. But it IS quite unusual to have them so early on as the
fourth month- it doesn't mean that there was anything wrong though,
just that either the foetus was growing rapidly early, or that your
uterus was pushed up to an unusually high position in your
abdominal cavity, maybe because you have a narrow pelvis.