I just went to a class on Saturday after not having been in QUITE a while and I fatigued my core fairly easily. Yesterday I was REALLY sore but I'm finally recovered.

I get the headaches too but didn't this time around. I think it's because of all the blood rushing in and out of my head from the different exercises. I did also make sure that this time I paid attention to not straining my neck. Maybe that did the trick.