Discharge help!

I’ve been taking this birth control for years. Back in December I stopped taking it because I was out and couldn’t get any for a few days. Then I skipped December’s period and had one at the end of January and took my pills on that Sunday. I haven’t had a period since and I’ve started to have brown discharge for roughly 3-4 weeks? I’m still taking birth control and haven’t missed a pill since I restarted. Any advice on what to do or why this is going on? (I have talked to my dr but she didn’t say much besides come back in 3 months if nothing changed)