Spotting?

So I am currently 4 days late. My cycles are not usually the same. My last period was sept. 1st and I had unprotected sex on Sept. 21st. Took plan b around 50 hours after and read that it would not work for me since I am overweight. I was fertile during the unprotected sex. Anyways, today I am experiencing cramps and have very light brown spotting. I took a pregnancy test 2 days ago and it was negative. Can you test after implantation or should I wait?