Problem with my little sister...Am I really embarring her?

She's in 7th grade and I'm a senior. we go to the same school. The other day I saw her in the cafeteria with some of her friends. I was in a rush to get to cl but I wanted to get a bottle of gatorade for practice (after school). I was in line and so was she. she was ahead of me. When she went to pay I saw she didn't have enough cash to cover the expenses of her lunch, I cut the line and went to her and told the register lady to add my gatorade to the bill and remove the skittles. I then paid for my sister and me and left for cl. When I was waiting for her in my car after practice to drive her home, she walked right p my car. I yelled at her out of the window and said "hey, kiddo, I'm here." She responded "I know" and walked away. I drove home and she was taken home by her friends mom. Later that night I asked her why she didn't come home with me. She said that I embarr her and she can't wait until I'm at college so she doesn't have to see me. Then she slammed her door! I never really talk to her at school, its a mutual agreement to act as if we are not related at school so she won't interrupt my conversations with my friends and vice versa. I just don't understand how I could have embarred her! I mean she should be thanking me for paying for her! Not having enough money to pay for lunch is way more embarring than having your older brother pay for you! Okay well now that I just read that maybe not, but still, why would she say she doesn't want to see me. Maybe I'm just being sensitive cause I'm overtired and stressed but I think she is taking this to far. Later that evening I called my big bro to discuss this with him. She is pretty tight with him since he's really nice to her most of the time whereas I'm not the nicest brother to her...but I know I should be nicer but its really hard. But anyways he said what I did was wrong, about the skittles, and that may have been embarring to her; the fact that I was monitoring what she was eating. I JUST DON'T GET IT!!! WHAT DID I DO WRONG??? Oh and by the way the real problem is WHY ISN'T SHE TALKING TO ME??? its been like 4 days since this happened and when i try to have a conversation with her or ask her about her day she just shrugs and walks away.