Freaking out :/

Had our anatomy ultrasound this past Monday and found out that we are having another little prince :). We were told at the appointment that everything looked good.
My OB called today and said the babies head is measuring smaller than it should be, compared to the size of his abdomen. I have to go back in 2 weeks for another ultrasound to remeasure.
This past week I was hospitalized for breathing problems and they found a blood clot in my lungs. I've been on mumerous meds and oxygen, on top of several control meds due to other health issues. My first pregnancy was so smooth and had no problems, now we seem to have back to back problems this time around. 
Has anyone else had this happen? Where the babies head wasn't measuring where it should?