Tough to say...
There are certain rhythm problems that only come on with exertion.
Palpitations however, are an unnatural awareness of your heartbeat, so even being severely deconditioned can do it. I'd find a cardiologist or cardiac electrophysiologist and have them run you on a treadmill while wearing ekg. If that doesn't work, we'd get you an "event monitor" to record the palpitations.