I don't think sending an anonymous note is a great idea...because it may eat away at you in the end, and be more hurtful to your massage therapist than you anticipate.
My suggestion is to find a more direct way that takes your actions and feelings into accountability, as well as hers. I'm surprised that this odor issue wasn't addressed for her in massage school, if it is indeed her. You don't have to come right out and say it is her because you're really not 100% sure that it is.
Maybe you can tell her you have allergies (or experiencing some new allergy/allergic reaction), and that there is an odor in the room that you are very sensitive to.
Jonathan wrote:...so Imph Sorfy. (That's how the apology sounds with my foot in my mouth.)