Levonorgestrel - Plan B and late period?

Have you taken a pregnancy test yet? I'm kind of in the same predicament. I was due 3 days ago, I had sex on the 15th, 6 days after my period. Bled the tiniest bit after about 4 days after and have been feeling on and off symptoms like nausea, cramping, burping, lower back ache but funnily enough, no breast pain which I usually get really bad before my period. It's driving me mad.

Have you done an early detecting pregnancy test?
If it's not pregnancy, then it could just be the huge dose of artificial hormone in Plan B. That can cause those symptoms, sometimes for weeks afterwards. Sometimes even longer, for some women.

Being so close to ovulation, I'd have opted for the other emergency pill called Ella (Ulipristal Acetate). It's the only pill proven can prevent pregnancy during the 24 hours around & during ovulation, itself, but even that pill is not guaranteed. Plan B might help in that situation, but there's no evidence. You can see that about Ella, on the Ella Manufacturer's website.
Unless you chart your fertility symptoms each day, you'll not know exactly when you're ovulating, as it's only a few hours within your fertile window. So it's impossible to know how close you were.
I hope that it was able to help.