She may just be sensitive or ticklish around her belly. Some horses are just like that. And yes, mares may kick at you when you touch their udders, it is something that you have to get them used to, as they need to be cleaned, but imagine if someone tried to randomly touch you in a private place? Wouldn't your first instinct be to slap them away? You just need to be gentle, but firm. Let her know you are there, and if she tries to kick let her know that's not ok, but don't just rush in, gently work your hand from her stomach back to her udders, just patting and rubbing her stomach as you work your way closer.