AF or IB?!

Hi all! This past Sunday I woke up and had light rusty brown discharge, which I didn't think much of since that's the day I was expecting af. The thing is, my "af" never got heavier like it normally does (and is over today) and stayed the rusty brown color. Could this have been implantation bleeding? I was either 12 or 13 dpo on Sunday. I took a hpt on Monday morning with fmu but it was negative. The only other symptoms I'm having are mild tingling in my breasts, sporadic mild cramps, headaches, and extreme exhaustion (and I mean extreme). Any input would be appreciated! :)