Hi Moth,
Thanks for reaching out, and it's great to hear that you've been reflecting upon your feelings and considering different reasons behind your emotions.
Although my experience is different from yours, I've had similar moments where I felt something more for a friend but it didn't seem romantic either.
As to the reason behind your emotions, especially considering your reflections, I feel it is possible you may be feeling (somewhat) sad, because your best friend will now be likely devoting much attention to his girlfriend, while in the past more of that time and energy may have been spent with you. In other words, it may be some form of jealousy or some form of loss that feeds into a feeling of sadness. I think this does not necessarily mean you have romantic feelings for your friend; rather, it is understandable that you have a greater attachment and expectation from your best friend of many years compared to most other friendships, potentially explaining why you feel it is not exactly platonic attraction either.
In summary, it's great that you're thinking about this, but I don't think you should worry too much about it. I've talked to other people about similar feelings, and it appears that it is relatively common to have feelings that seem to belong to neither romantic or platonic category for close friends you really get along with.
Hope this helps. :))
-Greenfern
