Excruciating back aches and sore 3 weeks after plan b :( !?

If you took plan b and had a negative test it's pretty safe to say you're not pregnant. Side effects of plan b pills are rare but when they do happen they are characterised by nausea, tenderness, fluid retention, dizziness, ireegular bleeding, abdominal pain, vomiting and headaches. So it could be the plan B, If it gets worse or stays the same than see a doctor.