A girl I coach started crying after practice? What does that mean? what should I do?

So I coach high-school volleyball and most of the girls are juniors and seniors about 15-17 year olds. Today we did one of the most difficult drills, I set them up to spike and serve the ball. Some knew how to do it, and others didn't but they tried anyways. So I finished practice with some push-ups and simple things. Everyone starts to leave after practice , and I see one of the senior girls that stayed behind. She came up to me and said "Sorry" and I said "Why?" and she said "sorry, for sucking. I suck. I'm sorry" and she was sobbing and crying. She did not do good in the drills and I didn't scream at her or anything because she's new to the sport but it surprised me to see that she was crying afterwards. She doesn't know how to hit the ball correctly, she keeps swinging the ball and I'm always telling her she needs to stop swinging but thats as far as I gone. I'm startled with her behavior. She works hard during practice , but why was she crying ? and what should I do? I told her not to worry because she was new to the sport and that we had time left to improve that nobody was good overnight and she seemed content. But I'm worried she continues to feel frustrated I suppose?