Ok, so I am going to try to say this gently, because I mean it to be supportive and to try and help shed some light on how your friend may possibly be feeling, ok?

If your friend says that she felt you were demonstrating some stalking behaviour, it might mean that you were becoming too dependent upon her. It's a big burden to find out that you're the only support someone has, and that they have major transference towards you. Your friend can't be your mother, and she likely got overwhelmed by how intensely you tried to force that role on her. (again, you and she will likely have differences on opinions in regards to this).

Perhaps you don't view yourself as having done anything, but it's clear that she feels you have. I know it feels and seems like she's done this abruptly, but there's also a chance that she's been trying to separate from you and has failed at establishing a healthy relationship/boundary with you. There may have been signs of this that you missed, because you were pushing so hard to make her into something that she can't be.

I'm sorry, I know that that is probably really hard to hear. And again - I don't know her, or you, or what really has gone on within your relationship. So I may be totally wrong, but it at least warrants consideration.

If you are young enough for your parents to still have control over your medical situations, then that means you are still a student (plus you mentioned exams). There should be a school counsellor that you can talk to?

Reaching out like you have here is a great step, and takes a lot of courage! You can form friendships online; some of my closest and longest-running friendships are online ones!