It really, really is VERY common. Infact, almost every flight has some medical incident occur onboard and certainly one in three if not more will see someone collapse.

That's why we as crew are trained quite highly in first aid, CPR etc.

Travelling is stressful and a lot of people fail to eat/rest adequately before a flight. Suddenly, reduced oxugen levels, standing up quickly after having been sitting down for a number of hours, excessive alcohol intake (on a fairly empty stomach) all compound to cause a collapse.

Don't worry about it. It's amazing how many people experience this for the FIRST time on a jet. Just be thankful that you had some of my excellently trained colleagues on hand to look after you (I assume you were on BA).