Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency care for severe difficulty breathing, blue or grey lips, fainting, confusion, sudden chest pain, or rapidly increasing swelling of the face or neck.
New leg weakness, loss of bladder or bowel control, or numbness around the buttocks or genitals also needs emergency assessment because it may indicate pressure on the spinal cord.
If you are being treated, contact your treatment team promptly about:
Follow the urgent contact instructions given by your treatment centre rather than waiting for the next appointment.
Arrange a routine medical appointment if you have an unexplained lymph node that persists, grows, or feels hard, especially if it occurs with night sweats, fever, itching, tiredness, or unintentional weight loss.
These symptoms have many possible causes and do not by themselves mean lymphoma.
The article also says
A fever during chemotherapy can be a sign of a serious infection even if you otherwise feel reasonably well.
Every sign above is quoted from the sofidex article on Hodgkin lymphoma. If you are not sure, err upward — it is always reasonable to be checked.