Infection?

When I went to the ER I got another pelvic exam and was informed I have an infection called vaginitis. The look on my face was probably hilarious this was a complete shock being that at my last pelvic exam about 5 weeks ago everything came back fine. The doctor told me pregnant women are more susceptible to infection and that this is normal and gave me medicine to treat it. I still felt really shocked like how could this happen? Has anyone ever experienced anything like this?