Hi Cheryl
There is so much to that comment! When you first start there is a TON of stuff you have to pay attention to. One of the possibilities is that your breath is not as deep or full as it could be: Because you’re a new yogini, AND because that attention is all about the sit-up (all those what and how to do the sit-up thoughts and attention). It’s highly likely that you’ve been holding your breath in class at times you are unaware of.
Also with perhaps a recent return to activity your body is just getting used to moving blood etc around. So don’t worry. It’s pretty normal. I promise you that in a very short time, the sit-ups will just be a part of the practice that you enjoy (well, even just not even notice). They are just what you do. 
Namaste
Gabrielle 
