plan B + late period ?

6 weeks ago I took plan B. One week later I got my period which was perfectly normal. However, my next period was supposed to come 2 weeks ago. And it's still not here. 3 weeks ago I had spotting for one day. At first I thought that maybe my period came too early but it was just one day and nothing ever since. Is it possible that the plan B messed up my cycle? I took 2 pregnancy tests (yesterday and a week ago). Both came out negative. Any thoughts?