Brown discharge

Hi. I'm feeling a bit confused about my discharge. I have pcos and I have been put on metformin. My last actual period was in may . I have not had a period since. Well on Thursday I noticed I had brown discharge. Period consistency but lighter and not heavy. It doesn't show on my underwear. Friday I had very light brown discharge, and today as well, brown discharge. I haven't noticed anything else accept that last week I was vomiting. But since then has stopped. I've taken 2 hpt last week and the week before, both were negative. Basically I just would like to know if anyone has experienced this before or if I should be concerned.