My horse bucks at me when i lunge her but she never bucks when i'm riding?

Hm. My horse used to do that when he was younger. If she doesn't buck when you're riding her, she's doing it in play. However it can be very dangerous and I was almost kicked in the head because of it. Next time you lunge her- put a lunge line on her even if you're in the roundpen. If she bucks at you pull her head in and make her stop (do not yell or reprimand her, simply make her stop and face you). It should be absolutely unacceptable for her to buck at you. It will be tiring but in pulling her head in to face you- she'll learn that it's not playtime. Be safe and good luck!