How is brown discharge IB at six weeks pregnant?

All I've read is implantation bleeding starts around your missed period and before you get a positive test. I missed my period on Christmas, took a test on 12/31 and it came back positive within 30 seconds. I took 4 more and the line became darker. But now I'm experiencing dark brown discharge. I should be 5 weeks, 5 days now. I did have a horrible vertigo experience Wednesday night and I discharged a little bit more than usual and got sick because of it. The next day I tested again and still a strong positive. The nurse I talked to said not to worry about the discharge. But seriously? How can you NOT worry? The only symptom I'm feeling is tired. Did I just find out early and the symptoms are on their way? I feel the bloat and heartburn and gas. This is my first baby.