I would definitely do the probiotics and pursue working on the digestion issues. However, I would also see a doggie chiropractor: www.avcadoctors.com
If there is an impingement of the nerves that feed the digestive system, it can cause symptoms, and often an adjustment will help. From the history, it sounds very likely that something is misaligned.
When I adopted my girl at 6 months, she arched like, and had little quivers along her back. But a few adjustments fixed her right up.