Can the cause of my palpitations be stress if I've never had palpitations due to stress before?

Yes. No one ever has anything until the first time that they do have it, right? Stress, caffeine, smoking, diet pills, cola, coffee, asthma medicines, cocaine, lack of sleep - all can contribute. Many times there's no reason at all. Palpitations tend to come and go. If it's bothersome, it's time to see your doctor.