What Causes Brownish Vaginal Blood After Putting IUD?

I have an IUD and i tend to spot drownish blood with a viniger smell Xp but befor i got the iud i got tested for STDs but that was a year ago i broke up with my old partner i had for two years and just got active with some one new he called and told me he got a rash on his "head" and last night i noticed my discarge was yellow ish and vinager. yesterday in the morning we had sex after ward i spotted blood stopped then yellow! gross we did not use a condom X( is it and STD, STI or Yest IN