Diarrhea no pun intended is a messy subject. Most of the time it is a self limited infection, caused by bacteria, viruses or protozoa. At times however, the diarrhea may be heavy enough to lead to significant dehydration and dangerously low levels of such critical electrolytes such as potassium. To make matters worse there has been a resurgence of a certain bacterial infection called C difficile which may lead to inflammation of large portions of the large bowel.
And of course not all diarrhea is the result of infections. Sometimes inflammation of the large bowel is the culprit. This is seen with inflammatory bowel diseases (read ulcerative colitis and Crohns) resulting from antibodies produced by the body against the colon. These conditions often present with bloody stools, belly pain, weakness and are best diagnosed with a colonoscopy with biospy.
Food allergies can inflame the bowel as well. High levels of thyroid hormone will increase the speed of the gut contractions and lead to diarrhea. In short there are many things which cause diarrhea.
If you have diarrhea and have recently been on antibiotics, gone camping and drank unfiltered river or lake water or have spent some time in third world countries, then you should see your doctor. This is because these are risks for bacterial and protozoal infections and will probably need antibiotics.
Much of the time antibiotics are not needed, but if you are feeling weak and or dizzy from the diarrhea, you may need to go to the ER, where your electrolyte levels can be checked and where you might need IV fluids.