I think I have had this before: when I was deep into Tai Chi Chuan, at one point, I had this same kind of pain. It reminds me of another kind of ache I used to get when I worked out a lot with weights back in my late teens. I did a bunch of weight lifting/body shaping and I remember this kind of pain in my legs and arms. It kind of felt good in a way, because you knew you were working out.
I did not and do not expect that from Qi work, and yet, I think I get it sometimes. I have an intuitive sense that this is a good thing. It feels a bit like doing a lot of sit ups only I am mostly feeling it in the lower abdomen and not the upper.
Just wondering if anyone else has experienced this. It is not anything terrible, just an ache. I will also search the archives.
But what say you? You few, you precious few... (BTW, anyone know where that is from? It sounds familiar to me, a poem or something, but I can't quite place it).