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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.