I think because your centre of gravity changes when you are pregnant, and your posture naturally changes a little to compensate for the extra weight out front, that can make it a bit harder to swim like normal. I mean it stands to reason that carrying an extra few kilos out front will affect your balance and movement in the water. Plus being sick and not exercising for a while is probably part of it too - your body is probably still recovering. I am sure it will get easier the more you swim!