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 if you faint, have severe difficulty breathing, become confused, vomit blood or have heavy rectal bleeding.
Seek urgent medical help for severe or rapidly worsening abdominal pain, repeated vomiting, fever with marked abdominal tenderness, or an increasingly swollen abdomen with an inability to pass stool or gas.
These can be signs of appendicitis, perforation or bowel obstruction.
During cancer treatment, follow the urgent-contact instructions provided by your oncology team, especially for fever or signs of infection.
Arrange a routine medical appointment if you have persistent abdominal discomfort, unexplained bloating, a lasting change in bowel habits, reduced appetite or unexplained weight loss.
These symptoms are more often caused by other conditions, but they deserve assessment if they continue or worsen.
The article also says
If you have already been treated, report new or increasing pain, abdominal swelling, vomiting, difficulty eating, bowel changes or unexplained weight loss.
These symptoms do not necessarily mean the cancer has returned, but they may need investigation.
Every sign above is quoted from the sofidex article on Malignant appendical tumor. If you are not sure, err upward — it is always reasonable to be checked.