Sewage smell from vagina during fingering. What's wrong with her?

I was fingering my girl today, and noticed what was like a sewage smell coming from her vagina. I think she noticed it too, because shortly after the smell got worse, she started pulling my hand away from her. After we were finished, I noticed her go to the shower and vigorously washing down there. Just want to add that she is an extremely sexually active female. Other than her, I'm not sexually active at all. I'm not too familiar with lady problems, but I do care about her and want to know what I can do to avoid this happening again. Any ideas what was wrong with her? Might she have an infection or STD? I really hope not. Any tips on things I can do to avoid this next time?