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 have severe or rapidly worsening abdominal pain, especially with a hard or swollen abdomen, repeated vomiting, fever, faintness, confusion, breathing difficulty, clammy skin, or very little urine.
Avoid eating or drinking while waiting for emergency advice because urgent surgery or anaesthesia may be needed.
Suspected secondary peritonitis always needs urgent hospital assessment.
After treatment, contact your surgical or hospital team urgently if you develop increasing abdominal pain, a new fever, repeated vomiting, worsening swelling, pus or bowel-like fluid from a wound, or a drain that suddenly changes or stops working.
Routine follow-up is appropriate for gradual recovery concerns such as ongoing tiredness, reduced appetite, bowel-pattern changes, stoma care, or questions about returning to work and activity.
A sudden decline is not a routine follow-up issue.
The article also says
Do not drive yourself if you feel faint, confused, severely weak, or unable to move safely.
Every sign above is quoted from the sofidex article on Peritonitis - secondary. If you are not sure, err upward — it is always reasonable to be checked.