Spotting on Depo shot

Hello, I have been on the Depo shot for about 6 months and recently (the first time ever) I've been spotting brown stuff for like 3 weeks. First it was a period, then the period went away. Then a couple days later ... BROWN STUFF. I haven't been having sex and I'm sure it's not an infection. Has anyone else experienced this? And if so, what should I do?