Plan b fail?

Jennifer

I missed my period this month, I tested and it was negative before hand. But I had unprotected sex while being 10 days late on my period. I took the plan b pill maybe 48 hours later. Now 4 days later I'm experiencing brown discharge and cramps on and off. The discharge is kind of sticky and stretchy but very little. Wondering if this is normal. Or does anyone think Its implantation bleeding?