I'm confused (again)

I've been taking the Beginner 2¾ class two or three times a week for a few months. Barre always ends with 16 single-leg relevés on each leg. Centre is packed with activity. After class I'm tired, but otherwise have no ill effects.

For the last couple of weeks I've taken the Sunday Beginner 2 class instead of the Saturday Beginner 2¾ class because of scheduling conflicts. This class is much less strenuous than the Saturday class. No relevés at the barre. No grande pliés. Simple, short centre exercises. Yet each Monday morning I've woken up with sore, stiff calf muscles that make walking difficult. Why?

I have confirmed, however, that I haven't totally forgotten how to pirouette. When I'm fresh I can usually manage nice, clean, single turns either en dehor or en dedans on either leg. Nothing that will get me a job as a professional, but ones I can feel good about. Part of the secret seems to be really getting my hip flexors warm and stretched before class. It's only when I get tired that I start falling out of the turns. 

Yesterday during grand allegro we did a simple exercise consisting of two sissonne fermée followed by a sissonne arabesque, finishing with a pas de bourrée before repeating in the opposite direction. It's a simple enough combination that I had time to think about how I was executing the steps rather than just desperately trying to remember what came next. Our instructor had specifically mentioned how the rear foot should close, so after the first or second set I paid special attention to that. Then I noticed that the student in front of me was holding the arabesque position for a bit longer, so I added that little bit too. When I finished the last set our instructor caught my eye and subtly nodded. That tiny acknowledgement felt like she's stuck a gold star for achievement on my forehead. :-)