I'm falling for my best friend and I hate it!

I think I am slowly but surely falling for my best friend and it is the worst that could have happened in this situation.

For context: My best friend and I have known each other for basically forever at this point. Basically since we were kids and never, literally never has it occured to either of us that we could be more than just friends. Neither of us wanted it that way. She was always like a literal sister to me.

Recently she has left her longterm relationship and since then, we have been hanging out and writing/ calling a lot more than we have in the last few years. All this would not be a problem until another friend of mine asked me (mostly jokingly though) if there was something going on between us and of course I denied it then... however... it is like this one short question had implemented a worm into my brain, that I cannot get rid of now. Now I am looking at everything from a different perspective and I can't get this tiny question out of my head that whispers "What if...?" I catch myself being jealous when she talks of other men, I smile like an idiot when she texts, and I am trying to find excuses to talk to her.

I don't want this! I don't want a relationship in general to begin with and least of all with the one person that I thought was fixed of what we are to each other. I don't want to be the guy that "everyone says not to worry about" and then jumps at the first chance he gets. I hate to be that guy and I hate to be in this situation. I don't want to be the cliché and I don't want to ruin my oldest and best friendship with something that is hopefully only a stupid hyperfixation.

This is truly my best hope... that all of this is just an overreaction/ hyperfixation triggered by the joy of "having my best friend back" and the stupid brain worm that the other friend gave me. Anything else would be nothing short of a disaster.