I have been in the exact same spot as you. I have friends who always ask me to watch their daughter. Usually I don't mind because they are good friends and I could always get them to look after my son in return, however while I was pregnant I was too tired, nauseous and grumpy. When they called to ask me to babysit I said "I'm really sorry, I would love to help out but I'm just not feeling up to it" and they totally understood and found someone else to watch her. It's REALLY hard but you just have to be honest with her and say no.