What Could Be The Brown Discharge Lasted For 1 And 1/2 Weeks?

About 3 1.2 weeks ago my discharge was brown and sometimes red . thought it was my period starting. then lasted for about a week and a half. then stopped. then it started again today, and i thought i started. but when i went to change my tampon there was just brown discharge (i hear its like old blood) is there something wrong? it did this one other time about 8 months ago. I have the mirena iud