Yeah, I pretty much agree along custer's lines. Its too early for you to assume she's committed/limited her sexual relatonships just to you. And, she really has no obligation in telling you of her other escapades, does she? You say sex with her has tappered off, yet you've only been with her a couple weeks ... LOL! Let's put it this way, if the novelty of sex with you was still strong, she'd still be going strong with you, so obviously she hasn't been overly impressed with your bed skills. You're probably just another "cock sampling" for her, as she's probably sampling a lot of cocks. Nothing wrong with that ... heck, us guys love sampling all the pussy we can get, don't we? Personally, I find each and every pussy a unique experience. I assume its the same for the ladies out there.
Custer gave you a definition of what a cuckold is ... and I prefer staying with that definition myself. However, cuckolding has taken on a life of popularity of its own over the past 8-10 years, so there seems to be a lot of opinions/definitions of what cuckolding is these days. Its now applied to females as well as males, and its applied to unmarried couples as well as married couples who are supposedly in monogomous relationships. Also, whereas a cuckold event use to cause stress to the otherwise committed relationship, it is now done to stimulate sexual emotions such as envy, jealousy, etc to relationships that might have become a bit stale or boring. Unlike swinging, where either party of a relationship has sex with other people, however, cuckolding singles out one party while the other remains monogomous ... usually its the female who's taking additional sex partners while the male partner remains committed to the relationship.
As to whether she's fucking other guys while seeing you, that's quite possible. Several ways you can find out. Why not just ask her, or taste her pussy when she's unusually wet & loose seem. I'm not sure why it is, but often, when people (males & females) have sex with partners outside their committed relationships, the cheating act itself raises their level of horniness and they often go home wanting more sex from their regular partners.
So, the question you ask yourself is ... would this stimulate you, knowing she'd just had sex with another man? If so, maybe you should share your fantasy desire with her. She might oblige fulfilling your fantasy and both of you will be happy. Just keep in mind, that women usually cuckold men because they need too, not because they want too. If you're not sexually satisfying her, you need to learn how or she's not going to stay around very long, trust me.