I had protected sex with my partner and now have white milky discharge what does it mean with little back pain not sure feels like cramps?

Unclear-See a MD: Probably worth getting seen and checked out. Could be that you have a Urinary tract infection. Seems hard to believe, but maybe a sexually transmitted disease. Could be just from hormones and progressing through your cycle. If you don't normally get back pain with your cycle, worth seeing a physician to be seen for this further.