For reference, I am starting my 13th week of recovery from LRTI surgery on my left (non-dominant) thumb. I have found that there are a few different techniques used for this surgery. In my case, my entire tendon was not used as it was "split" so that part of the tendon was used and the other part remains in place. I experienced some soreness in my arm, but no spasms as you describe. I had three small incisions in my arm to facilitate the pulling of the tendon. However, everyone is different and people have varying sensations of pain and ways of describing pain. I used hydrocodone (Vicodin or Norco) to reduce the impact of pain. If you don't have any problem with short term use of a narcotic, a prescription for that that may help you. Otherwise, depending on your medical history, Advil or Tylenol may be helpful to you. Also, be sure that you are not showing signs of infection, e.g. fever, discolored discharge from the wound, bad smells, etc. It is expected that you will experience some pain up front, but it will taper off. Did the surgeon install pins to stabilize the thumb for the next several weeks? If so, that adds a little more pain into the mix as well as another way for infection to get started.
In any case, contact the doctor's office if you have questions or concerns and follow their instructions. If needed, they can see you, remove the cast, inspect the wound, then replace the cast. This is something that is fairly common for them to do.
Let me know if you have any other questions. I am not a medical professional, but have had had many personal surgical experiences and worked with others who have had similar surgeries, so I only speak from my experiences. Always speak with your doctor for medical advice.
All the best to you.