PID and cervicitis aren't always caused by STDs. They can be caused by BV -
BV isn't sexually transmitted, but for some women, having sex can increase the chances of getting it. The vagina is really a delicate balance of good and bad bacteria, and sex can upset that. Also, if you are douching after sex, or washing with soaps/feminine washes after, that could be causing it.
This is a decent article about it - http://contemporaryobgyn.modernmedicine.com/contemporary-obgyn/news/recurrent-vulvovaginitis-tips-treating-common-condition?page=full
Basically, if you get recurring BV, try paying attention to what you are doing that might be causing it - douching, try using condoms, get your partner treated (though there isn't a lot of evidence to support that, but it may help and abstain from sex during treatment), wash with an unscented, delicate soap like Ivory or Dove, use unscented/unprinted toilet paper, etc. Some women can only wear 100% cotton undies.
I hope this helps and good luck! BV is no fun at all!