And if he's into domination and sadism, what happens if he goes after you? How big is this guy anyway?
Um...I'll admit to being far from an expert...but saying you're into BDSM is short-hand to let people know that you're kinky, and doesn't mean that everyone involved in the BDSM lifestyle is into the same thing, like sadism. Furthermore, it doesn't mean that all of the males involved are violent neanderthals. Also, having a Master doesn't always mean that you're sexual with him--sometimes, the Master is there for the purpose of training or play and there is no sex involved.
That said, Rebellion, MrFarFromRight makes some good points. If she's poly and she thought she'd never have another love, that sounds like an inconsistency. If she's poly, why is she having a monogamous relationship with you? Sometimes, less self-aware subs really will use outside relationships to anger their Doms so that they'll be punished. Crazy, but true, so are you sure you know your sweetheart the way you think you do?
A good Dom is going to want to meet you. Most Doms would want to know you exist before she commits to a relationship with you. Most Doms are going to want to have a say in whether the two of you can see each other, whether you're safe for her. Most Doms are going to want to know some boundaries, like, if they tell her to do something and you say something different, who is she going to listen to? You need to sit down with the two of them and hash out some rules--and you need to do some research into BDSM if you're really going to do this, because it *will* affect your life in some way. I can't tell you how often I've seen subs sad because their boyfriend is vanilla and just doesn't understand their needs. Also sad, I can't tell you how many times I've heard that unethical people will play at romance online and then leave their paramours heartbroken when they get bored, because none of it is "real" online, especially on BDSM sites.
I think my gut feeling is that there are an awful lot of discrepancies in her story, and maybe you're feeling awkward because your gut is telling you something is off.