Does this sound like ulcerative colitis?

Three days but you have bleeding, and then goes away again??? That is pretty unusual, as typical UC disease progression usually starts off mild and gradually keeps getting worse and worse, as the inflammation causes more damage to the GI tract... Are you sure you are seeing blood in your stool? Are you sure it's not red food? Or if it is blood, maybe polyps or hemorrhoids? I mean, that's just weird you'd only have little periodic 3 day attacks, but would have blood in your stool. As by the time you get to that point with UC or Chron's, you're not going to be recovering and suddenly go back into remission, because it usually takes several weeks or longer until the GI tract heals after a flare, which often times needs steroids or other medications to quell the flare. And short spurts of sudden bad GI problems, with no other known causes, like a stomach bug or food poisoning, would be more typical of something like IBS, except for the blood, which I don't believe is a typical symptom of IBS, but everybody is different. However if you are having blood in your stool, that is always serious as it can also be a sign of colon cancer, and yes, young people can get it too, and it's been on the rise in younger populations. And if you have blood that is visible to you, that likely means there's probably a lot more blood loss you don't see, so it also means you're likely losing lots of nutrients and could make you become anemic. I've been there, as I'm currently in a long time remission for my UC I've had for nearly two decades now, so if that's what it is, although it sucks, there's a light at the end of the tunnel, but it's a serious condition, and a life long condition, and no known cure, but treatments have come a long way since I was first diagnosed enabling long periods of remission, so not nearly as bad as is used to be.

It could also be something totally different, so you will likely need to have several tests done to know for certain. If I were you, I'd not put this off, and would suggest making an appointment to see your doctor as soon as you can, and if necessary, they may want to refer you to a GI doctor, or you may also just want to request a referral, as they know so much more about GI problems than PCP's, and they often take a long time to get your first appointment. Good luck. Hopefully it's not something too serious.