J.,
I know this may sound weird, but has your daughter had a BM in the tub? My daughter, now 20 months, is our "tub pooper". Unfortunately, we had a week not that long ago that she pooped twice in the tub in one night. The next night I went to put her in the tub and she flipped out. This went on for about two weeks. I kept reassuring her that the poopy was gone and all cleaned up and she has finally settled. We have also done showers - I have her in there while I'm showering in the morning and that seems to work well too.
It might be possible that she's relaxing so much that she feels the need to go poop and is concerned that she's not got a diaper on. You know, I think some kids really get relaxed to the point of getting ready to go and then they realize what's hapenning and it freaks them out a bit.
Just thought I'd share my experience. I think the bubbles and/or a new tub toy are a great idea.
Good luck.