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You've just been on holiday so maybe its just the air is different? And it was significantly colder this morning than what you were experiencing in Croatia. It's probably mostly down to re-acclimatisation. I'm always weazy when attempting exercise after flying. Maybe the AC and air pressure of the planes messes with your lungs too?
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Because the body has begun to detrain itself.
I had the same after ten days off in August. Normally I'd notice a slight loss of form for a couple of days, but this time it took almost a month to get back into decent shape. I was fine on steady stuff, but any intensive stuff was incredibly hard work. I thought it was because I was getting old, but you don't have that excuse.
Hopefully you'll get form back quicker than I did.
Why do I feel utterly shit on the bike after a break of more than 5-7 days? Been off the bike for 12 days and have zero top end and wheezing up hills.