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 difficulty breathing, chest pain, fainting, or coughing up blood.
These can be signs that a blood clot has traveled to the lungs.
Seek urgent medical care for new or rapidly increasing calf swelling, warmth, redness, or pain.
A ruptured cyst and a blood clot can look alike, and testing may be needed.
A hot, very painful, swollen knee accompanied by fever or feeling unwell also needs urgent assessment because joint infection is one possible cause.
Report increasing redness, drainage, fever, worsening pain, or new numbness promptly.
Arrange routine follow-up if the lump persists, grows, returns after treatment, or limits knee movement.
Review is also appropriate if pain continues despite activity changes or if there are symptoms such as locking, repeated giving way, or ongoing joint swelling.
These features may point to a knee condition that needs further evaluation.
The article also says
After aspiration, injection, or surgery, follow the care team's instructions about wound care, activity, rehabilitation, and warning signs.
Every sign above is quoted from the sofidex article on Baker cyst. If you are not sure, err upward — it is always reasonable to be checked.