I'm concerned my best girlfriend may be a lesbian that has a crush on me but I'm straight, what should I do?

Girlfriend. Be assertive so that u can preserve boundaries between u two and maintain good friendship.
Lesbian? Explain to her nicely that u consider her a good friend and always will and that u r straight.
Share your concern. You can tell her what you're thinking. If you are uncomfortable doing that then talk about your relationships with men and who you are attracted to or are seriously dating. Most likely when you do that her crush on you most likely will fade away. If not then you have to be direct and let her know you want her friendship but are not interested in a romantic relationship with her.