I have an underage girlfriend...

So the relationship falls within the acceptable limits of the creeper gauge (half age +7), and I assume that either you're both in HS or she is but you're post HS.
Basically you have to worry about two different things, legal ramifications and moral ramifications.
I don't know where you live, but age of consent laws vary by state/country. Additionally, there are exceptions/loopholes based on when the relationship started, parental permission, etc. Look them up for your region.
As for moral, the general concerns with big age gaps are that there is something abusive going on, either in terms of you manipulating them into wanting to fuck you.
I can't speak to your specific situation, but if you two are in an actual relationship and you're not just trolling for JB, you can find a way to make it work.

If you are afraid she'll accuse you of rape if you break up, then make sure you take things slowly, and that you let her take the first steps. If you honestly can't trust her enough to get over this hurdle, it is a sign that you guys' relationship is a little borked...