Being sick in the woods is a real cause for concern, especially if you happen to be in a real survival scenario. If you're out camping you should have something in your kit to help, but if you happen to be lost or have no equipment there is still a small glimmer of hope if you can leverage your survival skills.
Number one, stay hydrated. This maybe tricky if you aren't careful, you don't want to be treating your diarrhea by drinking pathogen rich water right? Assuming you're alone, and uninjured (diarrhea aside) you could try boiling water. There is an assumption of being able to start a fire with what ever is available to you, being without equipment the fire plow might be your best bet. If you can manage that without shitting your pants, you're in luck!
There are a lot of plants that may help you. Both mint and blackberry tea are safe and will treat your diarrhea and keep you hydrated. If you're lucky and you happen to be in an area with either and you're in season. Throw a few leaves in a small pot of water boil, consume, rest (if you can manage). We're going for the tannins in these plant to help us, so don't think the fruit of the blackberry is gonna help you here. The tannins act as an astringent which tightens up our mucus membranes which helps us retain water, essentially soaking up water right otta yer poo.
Also if you're lucky to be around pines, the vit C in pine needles can help lessen the symptoms of irritated bowels which can be the cause of vomiting and diarrhea. Same deal as the other teas, throw some fresh needles in a pot of water on the fire, consume. Being around pine can alsohelp with your fire starting, water proofing and food. ie Fatwood , Resin and Pinecones, pine nuts.
http://survival.about.com/od/8/a/Several-Survival-Uses-For-Pine-Trees.htm
Charcoal is another one that will help your stomach, not necessarily wood ash but charcoal. Its not too hard to make, but can be easier if you happen upon someones garbage.