21 yrs old with heart and digestive problems...?

There are some very good answers here. The first thing you need to do is get the tooth taken care of. Get it pulled if necessary. As others said it will affect your entire system, it's infected so get it out. Next find yourself a Dr, a GP would be good as they have had some training in just about everything and what they don't know they can refer you to someone who does. Your stool should not be black, most of the time that means blood in the intestinal tract somewhere or in the colon so a Gastrointerologist needs to be consulted and a possible scope of the bowels done. Constant constipation is not normal but neither is diarrhea. Your gallbladder may have to come out. As for your "heart" problems, it sounds too much like stress to me. You have plenty to keep your body stressed out with your constant worry over your bowels and what you think is heart trouble. So your GP can run a stress test and EKG and Echo is necessary to put your mind at ease. You may need a mild medication like Xanax to relax you until you calm down. I'm not saying you don't have a reason to be upset but it is upsetting your entire system. So first find a good GP, take a list of your problems and go from there and try to relax before you put yourself into a heart attack. One more point, your tooth. After 3 courses of antibiotics and no relief you may have Osteomylitis which is an infection in the bone where the tooth is, that usually requires surgery. See a Orthodontist or Oral surgeon, it will show up on Xray of your tooth or a biopsy may be needed to confirm.