Have ulcerative colitis. passing bright red blood since yesterday not much cramping, should i see my doctor? thank you

Yes: Do not delay. You can become anemic very quickly if you do not address the problem.

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.