Gynocological question... serious answers only please...?

I have been having this odd yellowish l discharge for a while now. Sometimes it is streaked with a little bit of blood. I have an IUD in place, so I am wondering if this is what is causing it. I am in a married, monogamous relationship, so I know it's not an STD. I have never had an abnormal pap, and have never had HPV, so I know it's probably not cervical cancer. The only symptom I'm having is this weird discharge. I have a doctor's appt. scheduled, I'm just wondering what this could be. Any help would be appreciated.