Short episode of AF or not?

Lying on my back a few minutes ago (I haven’t yet got up) I felt some fluttering sensation around the heart and experienced the heart beating irregularly. I coughed (deliberately) and the irregularity immediately stopped. The whole thing lasted barely 5 seconds. I am wondering if this is AF, atrial flutter or just some sort of ectopic activity. Not sure how long something like this has to run for it to count as AF. I find that this sort of thing tends to happen when I lying on my back or reclining in a chair, although it isn’t that frequent.