Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services for sudden severe abdominal pain, a hard or very swollen abdomen, collapse, severe breathing difficulty, or heavy bleeding.
These may indicate perforation, complete obstruction, a blood clot, or major blood loss.
And for anyone, someone who:
A fever during chemotherapy needs prompt advice because treatment can reduce infection-fighting white blood cells.
Follow the temperature threshold and instructions supplied by your treatment team.
Seek urgent medical care for:
Arrange a routine medical appointment if you develop rectal bleeding, a lasting change in bowel habits, unexplained weight loss, ongoing abdominal discomfort, or unusual tiredness.
These symptoms often have a non-cancerous cause, but they should be assessed if they persist or recur.
And for anyone, someone who:
The article also says
If you have already been treated, contact your cancer team about new symptoms, worsening treatment effects, changes around a stoma, or problems eating, drinking, or passing stool.
Every sign above is quoted from the sofidex article on Colorectal cancer. If you are not sure, err upward — it is always reasonable to be checked.