The most likely cause could be change in your refractive error, which could be subtle enough not to be detected on routine examination. You need to get a proper dilated examination, followed by retinoscopy and a post mydriatic test in order to get a perfect prescription of glasses. There may also be other causes of headache, which need to be ruled out by a physician.
Next Steps
Consult a qualified Ophthalmologist to get a thorough eye examination and a dilated examination.
Health Tips
Make sure you read in a well-lit environment, have a comfortable reading distance, and follow 20-20-20 rule to decrease eye strain.