Whenever two people have a conflict you should always look for the third party...the third person causing the conflict or "fanning the flames" so to speak. There is ALWAYS a third party. Always.
http://www.freedommag.org/english/vol36I2/page25.htm
In this case I think it's obvious that Sandra is the Third Party. It sounds like from what you said, Sandra is a "fake" and I would bet that she feels threatened by your friendship with Becky. I've had this occur all throughout my life. I would put money on Sandra telling Becky things about you, or encouraging small negative feelings Becky may have toward you until they are BIG negative things.
A friendship with three is always hard. I see it with my stepdaughter (if there are 3 girls, one always ends up excluded).
You can resolve this issue by talking with Becky (not Sandra). Your text probably made Becky feel bad, and I wouldn't be surprised if she mentioned it to Sandra and Sandra fanned the flames a little more and said "well you shouldn't feel bad" and "you have a right to invite who you want" and "Jennifer is just being horrible" and such things.
Just talk with Becky when you've calmed down. Ask her if you've done something to upset her. Tell her you value her friendship. Ask her if anyone has told her anything negative about you recently (you don't have to call Sandra out, that will just make Becky protective of her).
If maintaining a friendship with Becky is too hard with Sandra in the way, I think you could look to friendships that are less high-maintenance. Eventually Sandra will show her true colors.
Good luck!