I live in CT and each year in the Spring at the beginning of allergy season I suffer with severe diarrhea. Does anyone else find that they have diarrhea when allergy season begins? Could the diarrhea be the result of my allergies? I have been tested and I am allergic to tree, grass, mold, etc. (Most things in the environment.) I have terrible nasal drip. I use nasal spray, meds etc. Please help me solve this mystery?

This is only a thought: since you have lots of sinus drainage, it is possible the digestive system is also loaded with this drainage causing the loose stools.

Have you tried taking Mucinex? This is over the counter and helps to thin mucous. Might be worth a try. Mucinex is over the counter now.