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:
These symptoms may indicate a pulmonary embolism or another serious condition.
Seek urgent same-day medical assessment for new one-sided limb swelling, rapidly increasing pain or swelling, or redness that is spreading toward the groin or behind the knee.
Urgent assessment is also appropriate if clot symptoms develop during pregnancy, soon after giving birth, after recent surgery, or when you have cancer or a previous blood clot.
Fever, chills, pus or worsening redness around a cannula site may suggest infection and should be assessed promptly.
If you take an anticoagulant, seek urgent help for heavy or uncontrolled bleeding, vomiting blood, black stools, severe headache after a head injury or other bleeding that concerns you.
Arrange routine medical review for a tender, hard superficial vein even if you otherwise feel well.
Follow-up is also appropriate if symptoms are not improving as expected, keep returning or remain troublesome after the initial episode.
Every sign above is quoted from the sofidex article on Thrombophlebitis. If you are not sure, err upward — it is always reasonable to be checked.