My mom always keeps a Beta and she has each one for years... Every now and then she would get a little impatient about waiting for the water to get to room temp and her fish will inevitably react the way you described, heidi... So shock from water that's too cool could do it. I asked her about it when I read your post though and she said what she has noticed is that if she overcleans the bowl.. as in she cleans out the rocks and really gets it sparkling clean -- her fish HATES it and will mope at the bottom of the bowl for days until some algae grows back in. So now she keeps an algae-coated plant in the bowl with him at all times and he's much perkier. I am certainly no fish expert -- so I have no idea. Just passing along what Mom said.
If you have men who will exclude any of God's creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men. -- St. Francis of Assisi