I occasionally get palpitations and mild tachycardia (max 108) upon standing. no explanation after testing. could sedentary lifestyle cause this?

Probably normal: however there is a disease of the autonomic nervous system called positional orthostatic tachycardia (POTs) which could play a role. In addition the medications you are taking could cause this as well. Ask your physician to evaluate your autonomic system if this becomes a clinical problem for you. In the meantime hydration is key to management.