Well, there's one sure way to find out. Call Bella and talk with her. Tell her up front that you're confused and hurt by the ways she's been interacting with you, and that you really want to know what has caused her to act in this manner.

Your decision on whether or not to attend their wedding should be based on what Bella reveals. If she's forthcoming, great; perhaps you can deal with any issues she has with you. If her communication is cold, or distant, then you will know that your response to the RSVP will be 'non'.