Ever been pregnant? It gets pretty uncomfortable! I certainly felt nippy, 🙂 . The important thing is to remind her of her manners – I find making THEM back away from YOU *immediately* works very well. All of our horses are trained to back when we wiggle the lead rope or give a hand signal – the faster/more vigorous it is indicates degree of reprimand (but never hit them!). Horses remind other horses by degrees – from ears back thru to biting. Nipping can become dangerous, so definitely needs strong correction. Other replies have good suggestions too – do make sure she is healthy and comfortable. Best of luck!