Have ulcerative colitis. Passing bright red blood since yesterday not much cramping, should I see my doctor? Thank you

Absolutely. You don't say if you are on current meds for uc or what they are but if this bleeding is new and you are on meds, you need to speak to your doctor promptly to let him know your status and to decide if your treatment is not working or there is another cause of the bleeding. Don't delay.
Yes. Do not delay. You can become anemic very quickly if you do not address the problem.

Related Questions

Got ulcerative colitis been pooping blood increased my pills yesterday. I pooped 1 big dry red poop with mucus I think. Came out in an ellipse shape?

You need to discuss. This with the doctor managing your uc meds. You may need an adjustment in your meds or at the least he needs to follow your progress closely.
UC is out of control. Ulcerative colitis is a lifelong disease that when flaring can have serious medical consequences. You did not share with us the extent of your disease nor what medication course you're taking. Nevertheless, the fact that you are passing blood with altered stooling requires urgent evaluation by your treating gastroenterologist. Call him asap please.

When should I call the doctor when it comes to ulcerative colitis? I have dull pain sometimes on the left side of back. I just can't see inside...

If you have. Uc and develop new GI symptoms such as pain, diarrhea that is different or more frequent, or blood in the stool then contact your treating gastroenterologist for advice. If you don't carry the diagnosis but suspect it then make an appointment with a gastroenteroligist for exam and possible colonoscopy with biopsies.
Change in bowels. Everybody is different but you'll come to recognize your early symptoms of a flare-up. Usually moderate-severe abdominal pain and/or bloody stools are reasons to call your doctor.