It is unlikely that the number changed so quickly from minus to plus. Children have high accomodative capacity, which in simple language means their manifest number can change if they use their strong eye muscles. So basically, for a proper number checkup, it's essential to have a dilated/cycloplegic refraction done by an eye-specialist.
Next Steps
Consult ophthalmologist
Health Tips
Do not delay the checkup as it can lead to lazy eye if left untreated.