"I also felt she acted weird around me at the shower."
This is probably a reaction to how you were acting based on some false assumptions! On the shower, brides are not supposed to be involved in the planning. Their friends do this for them, and all the bride does is get their guest limit and then provide names and contact info for who she want invited. Obviously, I can't guarantee this is what happened, but it's how they're supposed to go and most people do it this way..
It's also untrue that she owed you a spot in her wedding because she was your MOH. All weddings are different. Just as one example, if her mom or her fiance's mom are helping pay for the wedding, they might have demanded input on wedding party.
At the very end, when you mentioned you don't feel close to her, 9 months ago she was your MOH! So why not ask her if you're imagining this, or are the 2 of you drifting apart? However, if you do this, do not bring up anything wedding or shower related. I don't think you're reading this right. She might ask what you mean, so have some other examples in mind. If you can't think of any, you're probably wrong about her being distant. Really, I think there's a good chance she thinks it's YOU being distant!