It could be any or a combination of the following:
1. Damage to the arm has been building up slowly due to your stiff racquet and strings.
2. You're getting older and your tendon isn't healing itself as quickly or properly as before.
3. Nvy, while soft, is not as soft as VS.
4. Your technique needs a tune-up, due to lack of practice.
5. You might be hurting your arm from an activity outside tennis.