What you are experiencing is very commonly described by pro cyclists. After a rest day in a stage race their legs feel heavy, they don't feel as good on the bike as they do when they have been riding the day before. As a result, none of them actually "rest" on the rest days during a Grand Tour. Instead, they go out for two or three hours, just spinning along gently to keep their legs in order.
And I've experienced it myself on long tours. After, say, six successive days on the bike, and feeling great, I take a rest day, and the day after that seems tough. It takes a couple of days to get back into it.
So, nothing to worry about, it's perfectly normal - though I confess I have no idea what the physiological explanation is. But you are probably magnifying the effect by going so many days without a break - after 10 or 14 days on the bike you may just need more than a day to recover. So if I were you I'd go back to your previous practice of taking a couple of days off per week. Remember, during a stage race the pros make staying competitive, rather than recovering properly, a priority - and as a result they lose quite a lot of muscle mass during the Tour de France, for example, which they have to make good afterwards. Your priorities should be different, in order to get fitter you have to rest.