It sounds like she is going pretty fast into poly. She should allow you some time to get use to the idea since it was not part of the original relationship make-up.
One worry would be trying to figure out what she is thinking. See if she is really into the poly concept or is using it as an excuse to try out other relationships while keeping you as a backup. I don't know her, so I have no idea where she is in all of this. But see if she is into all the poly stuff like openess, communication and focusing on the love.
I would suggest to go at you own pace sexually. You are not in competition with your girlfriend.