Hi Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern..
Intake of a Plan B pill or an emergency birth control pill can cause vaginal spotting & back pain. The vaginal spotting or bleeding you had must be
breakthrough bleeding which normally occurs within 3 weeks of intake of a plan B pill. This is normal feature of intake of the pill intake & you should not be much concerned about it.
The back pain you are having should be side effect of a plan B pill as it can cause back pain, lower abdominal cramps,
Tiredness, nausea, irregular period as common side effects. You could take anti-inflammatory medicine like
diclofenac for it
Don't use a plan B or
emergency contraceptive pill too frequently. Use condom or regular contraceptive pill regularly as method of
contraception in future. These methods have high success along with fewer side effects.
Refer to a gynaecologist for more queries
Don't worry you would be fine
Hope this information helps