I think people are overthinking this. She's not having sex with you when you visit her; she does have sex with the guy when he visits. That's just hurtful, period. You're not crazy. There IS something going on.
a) Maybe she's lost interest in you and it's time to break up.
b) Maybe she's lost interest in vag. There's nothing you can do about that. Hey, sometimes there are periods where I can't get enough cock, either. Doesn't mean she didn't want you at the start. Women's sexualities can be fluid, etc. etc.
c) Maybe she's lost interest in domming you. It can be a lot of work, and maybe she's not feeling it. Is she dominant with this other guy? Maybe she's not feeling the domme thing right now and the thought of having to do it is making sex daunting.