Repetitive movements, such as playing piano, are thought to contribute to problems such as carpal tunnel syndrome (CTS). The rapid and repetitive movements required when playing do pose a risk for problems such as you’ve described. This is not to say the feeling of weakness you report is from CTS.
Thumb weakness is usually not the first sign of CTS. Most often, people report feelings of numbness along the thumb-side of their hand when they awaken from sleep. Numbness and possibly even pain may spread into the palm and wrist and can eventually become a nagging ache into the forearm. Eventually, weakness may develop. Changes in sensation are usually noticed before weakness. Thumb weakness, by itself, may start for a variety of reasons, such as nerve pressure in the neck or upper arm. If your thumb weakness continues, you should see your doctor.