Does the arrhythmia feel like there is a longer pause between beats, followed by a heavier or harder beat?
something like beat...beat...beat...........BEAT...beat...beat
If so, you probably have premature ventricular contractions (PVCs) or premature atrial contractions (PACs).
I have the same issue.
Any time I change up my exercise modality and my heart starts adapting to something new, I will have frequent episodes of PVCs until my heart adapts. Then they go away.
You are correct...if you stay in shape year-round, you can keep them away.
I've been through years of cardiac workups and plenty of mental anguish over this problem.
If the problem persists or gets worse, you should get a 24 hour holter monitor performed so they can catch it on the monitor. They'll be able to tell exactly what it is.
Good luck. Send me a PM if you have any other questions on this issue...it's not an easy one to deal with and information is hard to find.