What lens is it? I'm assuming the kit lens. If so, most Nikon lenses, especially recent ones, will work with the D7100.
A word to the wise; bodies are cheap and lenses last forever. I would upgrade lenses before upgrading bodies. Of course, at the end of the day, do what makes you happy. :)
Any lens that worked properly on a D3100 will work properly on a D7100. The reverse is not true, as the D7100 can autofocus "screwdriver" AF lenses that the D3100 cannot, and can meter with manual focus lenses that the 3100 cannot. Any "AI" or later lens will work. That capability is one of the main reasons for getting a D7100, as it greatly increases the range of lenses you can use.
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A word to the wise; bodies are cheap and lenses last forever. I would upgrade lenses before upgrading bodies. Of course, at the end of the day, do what makes you happy. :)