Today I can really feel the core workout I did yesterday. I feel it kinda like a tightening tire around me. On my back it feels like minor lower back pain, but I know that's strengthening me so that I'll suffer less from back pain.
It doesn't really hurt, it just feels awkward.
I felt it last night too, and because of that it was hard to get to sleep.
I'll get used to it and I'm sure by sometime next week or at the end of next week it won't be so bad.