You really should see a physician. That rash may not just be a rash. There are some gastrointestinal issues that can present with a rash. One example of this is Crohn's disease. I'm not saying you have Crohn's, just that it is one GI issue that can present with a rash (http://skincare.lovetoknow.com/Crohn's_Disease_and_Skin_Rash). There are others. The fact that you have blood in your stool is a big sign that you need to see a physician. Along with the stomach pain and the rash, I am concerned that you have something going on that needs treatment.
Please make an appointment as soon as possible with your doctor to discuss testing and treatment. Please do not wait. Let us know what happens.