OP, happened to me exactly this week. two weeks ago got new irons and played a round with them; i was terrible. last monday, tuesday, wednesday i went to the range every day, the irons came around. played thursday, saturday with significantly better progress (shot a 91 on saturday, previous best on that course was 106, it is very very tough since i'm normally a mid-90s player in general). i was really excited.
i played again on sunday and everything was going downhill a little bit, and finally capped it off with a range session on tuesday and it was absolutely terrible. fat shots as you mention and those shots where you feel like your hands shift in the downswing and you shank it right. i couldn't figure it out, so i just left.
anyway, i think it is due, at least for me, to overplaying. i've never played that much over consecutive days. i eventually got impatient and honestly just tired of playing so much. i could never be a pro golfer. not to mention i have golfer's elbow and after the range sessions last week it really began giving me fits. i was/am icing it every day and taking ibuprofen. an injury--even slight--can make things both mentally and physically exhausting pretty quick. if i were you i would just take a few days off. i'm confident when i play again on saturday i'll feel better and more confident.