To answer your question I think I need to explain a little about clots.
A clot in your lung will usually first form in your lower leg. A small piece of this clot breaks off and travels through your veins and onto your lung. In your lungs your veins get progressively smaller and smaller until there is not enough room for the clot to continue and that is where it stays blocking off the blood supply. So it depends on how big the clot is to how far it can travel into your lungs. The smaller the clot the further it will travel into your lungs. A very small clot will only block very small veins and thus only a very small amount of blood. A large clot is the scary one. It blocks off quite a large amount of blood from a large area of your lungs. In your lungs your blood picks up oxygen and puts carbon dioxide out. So if the blood is blocked then it cannot pick up oxygen. Therefore the more blood is blocked the less oxygen your blood can pick up.

So on to your question.
Once a clot has got into your lungs things are very unlikely to get worse. A very small clot would probably not be noticed. A little larger clot would cause some shortness of breath and lower oxygen levels in your blood. Therefore your breathing rate and pulse would be faster. A large clot would cause great difficulty with breathing. Now if this large clot breaks up and travels further into your lung your breathing would become easier.

Probably the best test for clots is to do an ultrasound on your legs and as you know this is quite safe during pregnancy. Should an X-ray bee needed then they would put a lead blanket over your tummy to protect your baby.