What Causes Irregular Menstruation?

I have had the paraguard for 3 years now and have had regular periods. At least 3-4 days after my period was over and done with I started to have brown colored discharge and when I would use the bathroom there wouId be drops of red blood in the water and when I wiped. I spotted like that for about 3 days then it quit. What could be going on? I ve never had any problems in 3 years so I found that to be weird.