Implantation bleeding - how long does it last?

Amy • 33, married, Baby boy after successful IUI
I am 14 DPO and have noticed a small amount of brown/rust colored discharge when I go to the bathroom. Nothing significant. It started at 10 DPO and has been intermittent over the past few days. Could this be implantation bleeding? AF due tomorrow. Took a test yesterday and it was negative. Trying not to get my hopes up 😕