Hi there

There can be many reasons other than an STD for bleeding in between periods.  Actually your contraception method might be one of them.

Here is the official NuvaRing site with their questions and answers and they state that:

irregular vaginal bleeding or spotting (can be a side effect of taking this medication)

If you look on yahoo questions you will see that you are not the only woman who has experienced this side effect and particularly after intercourse.

I would stongly suggest that you seek the guidance of your gynecologist on this.  When is the last time you have had a Pap smear done as well?

Let us know how you are doing in the weeks to come.  I hope you get this resolved.