You'll have to forgive me if my response seems drawn out, hazy or watered down. Work has been taking a lot of my brain power as of late.
It's hard to really determine a cause, or determine a course of action in response to hers, because the person that the problem revolves around is her, not you. If a problem revolves around yourself, you know or can learn all of the inner mechanations to help yourself get over it, deal with it or at least hide it until you're ready to fix it. With cases involving other people, it isn't nearly as simple. You have to decipher her intentions before you can respond to it in any way, or at least make an educated guess.
These are a few things that may have caused her to take this course of action. She could legitimately like you, and is trying to find some way to make you happy. If this is the case, she's doing so very subtly. Now, since you said bus, I can only imagine you're in school. From my personal experiences in high school, girls and boys aren't generally that deep or subtle about their feelings. Those who are, bottle it all up inside. Does she seem to come across as one of those people? Likely not. She would either be disconnected from her peers, or a little too shy to talk to you at all. Did you feel like she came across as shy? Did she stutter, mumble a bit, or were her words clear?
Another reason she might have done it is because she actually thought of you when she saw it. If she saw you eat it once, and she may not have seen many others eat it, it might have stuck with her. Mentally, she may have been curious about the food. She might have stopped and thought about it, for whatever reason. This is why things stick with me, because I get involved in its existence, and why things are happening around it. If this is the case, then she probably is more interested in the snack and why you like it than she is you. However, that doesn't mean she's not interested in you.
What should you do? Well, I'd like to state an opinion followed by a few courses of action. I think that being anti-social does lead to some terrible problems later in life. I was a little anti-social in school, and I find myself having emotional issues. I'm extremely afraid of being alone, even though in high school I didn't want to be around others. There's a certain happiness that others can help you find in yourself. Without the happiness of other people, it's very hard to find happiness in yourself.
I think it would be good for you to tell her you appreciate the gift. Don't really think too hard about it, just thank her. She may start talking back, and I believe you should try to have a short conversation with her. I don't expect you to run around having parties, it's something you slowly pull yourself into. You have to make sure you're comfortable with moving forward. Don't ask her any questions about the gift. It's best not to look a gift horse in its mouth, and you don't want to let her know you might assume too much, and scare her away. This will lead to her telling friends, and they will likely scorn you as well.
Of course, this all tied into your earlier problem might make things a little more difficult. My suggestion is that if you ever have somebody interested in being with you, tell them up front. You don't want to let them figure things like that out. They need to understand that not everything is rainbows and unicorns.
This was such a terrible attempt at helping -w-