4 Year Old Potty Mouth

Hi ladies! I need help. I have a daughter who is about to be 5 in August . And, while, she can be a challenge at times, (like any child),she is a pretty good kid. She can be very hard-headed, like both her parents, but I would consider most of the things she does very normal for a child her age. We recently signed her up for an indoor soccer league for summer ,as she wanted to try it. Last night was her 3rd game, and after the game, her coach, (who is maybe 15 years old or so) walked her over to us and informed us, infront of her, that sometimes Gracie has what she referred to as an "attitude problem". I asked her to be more specific because I wanted to know if she just wasnt' listening, or if she was saying specific things, etc. She told me she called someone named Anna, who I assume is the other coach, a "stupid, dummyhead" and called her a "toilet" and some other things. While there are worse things she could be saying, I am obviously mortified that she would think its ok to say that to ANYONE much less an autority figure. We were supposed to take her out for ice cream after the game, which we told her she no longer got any ice cream. We went ahead and went because we had told our other daughter we were going, so we went ahead and went and let our other daughter get some, but did not let Gracie get any. When we got ho me, she did not get any other snack before bed and I made her go to bed early . I did talk to her about WHY she didn't get ice cream and had to go bed early and told her I wasn't sure what else her punishment would be because I wanted to think about it some more. My husband and I decided next week we are going to walk her over to make her apologize to both coaches for not being nice because last year at he daycare at the gym we work out at, she did something unacceptable one day there and I made her apologize to the teacher and she was super uncomfortable and embarrassed and have never had a problem since.
I guess I just want some advice because I am so shocked she would say those things. I know she knows better and I think she was probably trying to be silly, and just got carried away. We do not talk that way in our house, and I guess it makes me feel like its a refelection on me and I am a bad parent. It's really frustrating. I think I am shocked too, because other than normal sibling fighting that goes on from time to time, I have never heard her say things like that to other people and I have no idea where she would even get that. Anyone else ever dealt with this? Just wanted advice. My mom taught kindergarten for 30 years and I always remember stories she had about disrespectful kids or things kids would say at school and I always remember thinking "What the heck are the parents doing?" so I think I am overly sensitive to making sure I don't have THAT kid.
Any thoughts or comments are welcome! Thanks ladies!