I agree with graystreet about the possible misunderstanding the instructor may have had, that because you're trying to give her something, maybe you're showing your interest and will expect something in return. It could also be against the gym policy to accept things from people, who knows.

I really can't say why you're having a hard time with your friends. There aren't a lot of details in your post and there are so many things that could be going on. However, I do think you were way too honest with that person who asked you what you do with friends and that some kind of evasion would really have served you better. You could have said you're really busy with work right now, or that you're new in town and haven't met anyone, or that you're a bit of a loner, or you could have just shared something that you enjoy and ignored the part about friends. That person was reaching out to you. Who knows, maybe you have a common interest? And if not that person, then maybe the next person who asks you some question. I think it's important to make a good impression and not be overly honest with strangers.