If this all seems a little random, you may also want to consider (as CAMom said) an illness or a growth spurt. DS was mostly happy, but would have these really un-resiliant days/weeks and then would shoot up an inch or suddenly be able to do things he couldn't do before. I think some days the body just has other plans for the energy that might otherwise be devoted to learning.
I'll second this! GS9 has been 'off' the last few weeks. He's grown an inch during this last 4 weeks, a whole shoe size in 2 months, and gained 5 pounds-all muscle.
