I am sorry to hear you are worried about Tessa. My name is Rebecca. I am a veterinarian and will do my best to help.
Since she is 12, and the pain medication helped while she took it, this may be chronic joint pain or arthritis. If you want to know more, Xrays might be helpful.
You can try prescription anti inflammatory medication made for dogs such as carprofen or meloxicam, and it may help to use a pain medication like tramadol along with anti inflammatory medication. You can only get prescription medication from a veterinarian who has seen and examined her.
Aspirin may be helpful: 1 regular 325 mg aspirin, with food, per 70 pounds of body weight,once to twice daily. Do not use Aleve, or Advil/ibuprofen.
Supplements that may help are fish oil, glucosamine, and an herbal product called Doggone Pain (DGP).
Let me know what other questions I can answer.