What is the difference of the two words?
TheFreeDictionary.com states that regime refers to a regulated system in a diet or in an exercise. Regimen has the same meaning with the words 'intended to promote health or achieve another beneficial effect' added to its definition.
Which, then, is appropriate in this sentence? The masseuse continued to knead her body, and the woman did not wonder anymore why Thai massage is part of a normal health (regime, regimen) in Thailand.