Particular scenario with Schengen Visa timeframes

[DocPooki]

I'm an American currently working in Prague. I have a working Schengen/Czech Visa that expires May 31st, 2014.

I originally flew into the Schengen Zone on August 2nd, 2013. Eventually got my working/living Czech Visa mentioned above.


I'm planning to travel Europe this summer, probably starting in June, lasting until sometime in August (or so). I do not intend to work or live anywhere in the Schengen Zone this summer. Come May 31st, 2014, my Visa here in the CZ will expire -- at that point, will I need to get a stamp on my passport indicating the 90-day time period has started for my tourist visa? Should I fly out of the Schengen Zone come June 1st (or before) to get that stamp, or... what? I can't help but think that traveling the Schengen Zone with no indication of when you got here is a good/viable idea. Will my expired working/living CZ visa be good enough?

This is all fairly new to me. My girlfriend is British so I might go fly to her place at the end of May/beginning of June, and when we get back to the Schengen Zone I'll get that stamp. But... if I don't fly out of the Zone come that time, what should I do?

Thanks.