An occasional ache does not automatically mean the joint is severely worn. In the earliest stage, cartilage is largely intact and the X-ray changes are mild. Many people notice stiffness after sitting, or an ache after a long walk, and still manage well with activity, a healthier weight, and simple pain relief.
The grade is judged from your symptoms and an X-ray, not from pain alone. If the ache starts to limit walking, disturb sleep, or no longer settles with rest, it should be assessed in the clinic.