Polyamory should not be used to fix flaws in a relationship.
Polyamory is not a trial relationship!
Poly works best if the starting relationship has a solid ground, ESPECIALLY if not both/all partners are famililar with polyamory
Communication is the key. Lying and omitting information is the acid that dissolves polyamory
Not all polyamorous "couples" let each other date whomever they choose. Many have some kind of boundries or rules, and these should be discussed PRIOR to embarking on any dating. All parties are responsible for initiating these types of discussions
Monogamous people handle relationships with poly's in different ways. Communication is even more vital then when all involved are poly.
It does not matter what motivation she had for embarking on the relationship (probably she just wanted the guy...). What matters is her conduct towards you, and your motivation for allowing her to start seeing others. This is not a do first, ask later-type of situation. She just seem impulsive, and your rom-mate may very well have gotten your permission directly from you.
You are not causing drama. They are. Take a healthy break, and get some new influence in your life.
For info on poly, I recomend "Upening up" by Tristan Toarmino. Mono-poly is one of her many subjects in the book. I re-read this book all the time.