I had a bad practice. What is the culprit?

Today, my coach gave my team this main set: 3 rounds of 4x125s IM then 4x75s. The IMs have a cycling 50, starting with the first stroke of the IM (fly) and moving to the last stroke (free). The 75s went from fly-free-fly by 25, to all back for the 75, then finally all *** by round. 2400y. Right before that, we had a set of 8x100s flutter kick w/ board on 1:40. I've been getting better and better at kicking. On most days with this interval and number of 100s, I usually go 1:30. On a really good day, I can go 1:25 for several consecutive 100s. Today, I sputtered out halfway through, and then basically missed the interval on the last three 100s. I didn't flame out on the first few or anything. I paced myself well. I shrugged it off and got ready for the main set. From the very first 25 of the first round, I knew something was wrong. After the first four 125 IMs, I officially declared to myself I hadn't felt this way in the water for a pretty long time, since a Friday last July as a matter of fact. There are different ways to feel bad in the water. You can be sore and tired but still muscle through. You can have been out of the water for a few days and feel like your arms are slipping through the water when you come back. You can have been out of the water for multiple weeks and feel like you just started when you come back. Today, it was like my muscles evaporated. It was a very unique, specific feeling in the water that I haven't had once since that random practice last July. Until, that is, today. As of today, I have been swimming more consistently than I have been in a year and a half. I did not feel sore or tired, even with the increase in yardage that started last Monday (6350, 6000, 6600, 5600, 7000, 8050, 6600, 6000- excluding Sunday which I took off). But this happens. It's the second time it has happened and now I'm really peeved. As I usually do when I have a bad practice or a bad meet, I assign blame to some thing(s). "I did bad because I did or didn't do this..." Usually, I'm able to pretty quickly answer the question definitively, "What Went Wrong?" But not today. Today, I have a number of possible reasons, but I haven't settled on any yet. Here's where you guys come in. I need your advice so I can avoid this happening again in the future. Feel free to ask me questions, too, to jog my memory. Suspects: 1.) after practice, my coach said it could be because we haven't done very much fly lately and that affected the rest of the set. There are two problems with this: a.) we did fly in warm-up yesterday and that didn't knock me on my rear for most of practice, and b.) that doesn't explain the kicking set before the main set. 2.) after I was kind of expressing my skepticism about it being the butterfly, my coach also said sometimes there's no rhyme or reason, it just happens. Sometimes, for some mystical reason, practice and/or a meet goes bad. Obviously, I don't like this theory because it has an element of extreme uncontrollability about it. 3.) I have this leftover cake from the weekend in my fridge. I didn't eat a whole slice of it before practice or anything, but about an hour before practice started, I took my finger and filched the last of the frosting (which was enough to cover two digits of my index finger). I remember one of my former coaches telling me that simple sugar is the only food that has an immediate effect on your performance. On the other hand, was that little bit of icing really enough to do that to my practice? 4.) Somehow I might be tired and broken down despite not feeling tired and broken down. I had a great practice yesterday that I just absolutely killed. Not to mention, I had Sunday off. I am also a religious low-fat chocolate milk drinker and I have had a full night's sleep every night since before Saturday morning. One last thing and then I'll be ready for your :2cents: Before practice when I was sitting on the deck thinking about what kind of practice I would have today, I actually remembered that practice last July that was really bad and inexplicable. I don't care to say that I jinxed myself by remembering that practice or that I somehow accidentally programmed today's practice by thinking of that ancient practice, but I do wonder if I somehow subconsciously knew that today's practice would feel like that one last July and thus it wondered across my mind. Alright, what do you guys think? :)