What Causes Sour Vaginal Fluid Along With Bleeding Between Periods?

I have notice that my vaginal fluid has a sour taste, its been like this for a week. I just recently had a normal pap and tested neg. for all std's. I also have been having bleeding between period for about a week at a time. light but steady. not sure what is going on. I have a copper iud ( for about 3 years. I was told that iud can cause some spotting, but this is more than spotting, it a light consistant flow for a week off cycle.