How fast is your pulse during these episodes?
Have you seen your doctor about them? Thank you.
There is a condition called paroxysmal tachycardia where the heart speeds up occasionally because of heart conduction abnormality. However, the heart rate during one of these is far over 100, so that's probably not the cause.
Otherwise, shortness of breath, weakness, feelings of nausea, some fast pulse are not too unusual during pregnancy due to all the changes in the body and circulation system. Usually that is helped by staying well hydrated and maintaining good nutrition.
Just in case you do have paroxysmal tachycardia, it could be detected by wearing a heart monitor for a day or so. I would mention these episodes to your doctor for further evaluation.
Your doctor will also probably check your hemoglobin, since anemia can cause many of your symptoms. Clexane is a blood thinner, and it might possibly cause some internal bleeding which might result in anemia.
So be sure to drink plenty of fluids and eat well. Most likely there is nothing so serious that it can't wait until your next doctor appointment. Best wishes.