Im confused

I have a very good Friend. Shes a few years older than me. We meet over 1 year ago and have been texting regularly for almost 5 months. Shes like an older sister to me and she also said that she feels that towards me too. I never had a older sister and its something i always wanted. Shes just an amazing Person. She doesn't judge, tales time and is so whise and patient. I just like texting whit her. We are planning to meet up for her birthday, which would be the first time we see each other again in over 1 year. Thing is im confused about my feelings. They aren't romantic but im confused why im having strong opinions about that relationship. Like we don't know each other that long but she is very relatable. I fell understood. But I can't shake a curtain feeling of that something is off. Not about her but something else.I just don't know what it is but i just fell something is gonna happen or os already going on and that this relationship might break off.