Quit methadone after 12 years, reduced to 5mg, on two meds Lisnopril 10mg and Amlodipine 10mg am rapid heart beat and chest pain upon climbing stairs?

R/O Cardiomyopathy . You have not provided the results of the CT or stress test. However, these symptoms may be due to an enlarged heart. I am presuming that the Methadone was for maintenance. If you have swollen legs/ankles you may need to see your PCP for further tests, such as an ECHO.
Call cardiologist. The medications you take are both for very similar purposes. The provider may need to modify the doses or change in some way. Together it is possible to lower BP too much for certain activities - i.e., climbing stairs. Call ASAP - hearts are important and we only have one. I don't think this relates to your quitting methadone - but ask while you are there. I hope becoming freer of drugs feels good.