There is a small chance but its minuscule. Plan B pills can have side effects that's why they are a last resort. They can play havoc with your periods.
You have two choices, see your doctor or wait it out.
I'd do another pregnancy test (a different brand) if its also negative then wait and see if your next period is more normal.
If not, then you'll need to see your doctor even if you aren't pregnant.
Plan B side effects. What you are experiencing can be normal. Retest the pregnancy test. Do it first thing in the morning before you drink anything.
I think you are getting the heavy side effects.