I'm very sorry to hear about your loss and your confusion, it's such a strange time after a break up and I can relate.

6 months ago and I went through a phase similar to what you are describing after I had just gone through a break up. I too was considering whether or not I still had feelings for past lovers and I would get confused about my feelings upon meeting new women.

Whether or not this is what you are experiencing I don't know, but regardless, here is what I think:

Because the break up has left you are in a very vulnerable state you are also in no condition to be trying to figure out your feelings for other women. Be patient and time will tell you if these thoughts about your best friend/ex lover are thoughts worth having. For now I think it'd be best for you to keep it out of your mind until you've healed from your break up.