I can relate to some of what you wrote in your original post, particularly wanting to be alone and not liking small talk (also was the “shy/weird” kid at school).
I was diagnosed at 17, by a psychiatrist my mum took me to see and I had no idea why she did so. I think your best bet is to see a professional if you can, and ask their opinion. The autism self tests can also provide an indication, if you haven’t already done one.