Have you tried metamuclie. I have IBS (irritable bowel syndrome) and it seems to have, sorry for the phrase "thickened things up"....
Have you tried some "pro-biotics" such as ACIDOPHILUS/BIFIDUS? The antibiotics probably killed off these beneficial bacteria, and you need to re-populate the large intestine with ACIDOPHILUS/BIFIDUS, which hopefully will stop the diarrhea. Don't overdo, though, as overdoing can cause loose stools!
It sure does sound like you might have cought an intestinal infection from possibly contaminated dental equipment!
I have read that root canals can often remain infected, and be a source of continuing infection in other areas of the body!
Some people have to have the root canaled tooth pulled, to help end the problems that are related to still infected root canaled teeth.
Read other posts here about various causes of diarrhea, and read books at health food stores about causes of diarrhea. Share this info with your docs, to help you solve this problem!
I just found a wonderful drug to help with diarrhea. I have had a problem for six years! I.B.S. A friend found out she was not digesting her bile salts and she now take a 1/2 packet of Questran each night. Ask your Doctor about this.