What could this be? TMI photos in post

Hello I track my cycle using an app and on the day it expects my period I always get it. I had a uti/kidney infection almost 2 weeks ago. Now I am 3 days late: the 1st & 2nd day I was spotting light pink a couple times throughout the day. Today I noticed I now have brown discharge as shown in the picture. I do have little backache & headaches. I did take a test 4 days before my expected period with the 88cent test from Walmart but it was negative. When I took one from equate brand the 1 line was faint. Any opinions would be great!