First, are you certain she is seeing another guy or is that just an assumption? If you don't have any proof, it's best you don' think too much and create a problem that isn't there. On the other hand, if she is "actually" seeing another guy, she could be hiding him because things between them are not serious other than physicality. She could still have feelings for you.
And my best advice, it's best you move on since you guys are already exs months ago.
She may still be contacting you because she feels a friendship with you or still has feelings for you. The first is probably most likely. She may not be telling you about a guy she is dating because 1) She thinks it will hurt your feelings, 2) They aren't serious, 3) She is not in a monogamous relationship (playing the field instead), or 4) There really isn't any guy and you are simply feeling jealous because you miss having her around/as your girlfriend.
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