B.S.
Yes, it's from potty training. She is learning control. Lot's of kids do this. My granddaughter was especially bad about it. She was an extreme case, and if you really want the details, you could e-mail me, but trust me, she took this holding BM's above and beyond the norm, and we had to get really tricky with her.
I began rewarding her with a small glass of Dr. Pepper when she went poopy. She loves Dr. Pepper, and it was a treat she wasn't allowed very often, so she would practically do handsprings to get it. So, if she went poopy, she got a glass of it. In the meantime, we were sprinkling Benefiber (per her doctor's instructions) in all of her food to help keep her movements soft and easy to pass. After she got pretty good at going poopy for the Dr. Pepper, then I start switching the routine a little bit. I started telling her Dr. Pepper would help make her "go". It didn't take very long before the psychological benefit kicked in, and if she hadn't gone poopy in a while, I could give her a glass of it and tell her "Now remember, this is going to make you go poopy, so be ready!" Sure enough, she would drink it down (sometimes not even finish it) and have to run to the potty to go poopy ! She is 5 now, and it still works.
For those of you that are worried about lying to the child, it's not a total lie, Dr. Pepper always make ME go ! LOL. It really does ! And I would never lie to a child.
So there you have it. Maybe that will work for you, or maybe you don't need anything that drastic, but it worked for us.
:o)