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.