If you have rapid heartbeat and low blood pressure, call 911! (Internal bleeding)
My gastroenterologist has a "stool chart" which shows different shapes and colors of "stool". I just searched google.com for the words stool chart and found many charts under images.*
Also at the drugstore you can buy a kit to test for blood in the stool.
* I also saw a coffee cup with a stool chart on it - I think I will pass getting one of those!
http://blogs.scientificamerican.com/...dge-Sundae.jpg