Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Emergency care is appropriate for severe chest pressure, difficulty breathing, fainting, major bleeding, or rapidly increasing swelling after a significant injury.
These symptoms may have another cause and should not be assumed to come from breast tissue.
Seek prompt medical assessment for:
Routine follow-up may involve watching for changes in breast size, tenderness, or firmness.
A clinician may repeat the breast examination or review whether an underlying condition or medicine has been addressed.
If observation is chosen during puberty, follow-up can confirm that the tissue is stable or improving.
Arrange a medical appointment if breast enlargement is new, continues to grow, causes ongoing pain, appears after starting a medicine or supplement, or is accompanied by symptoms of low testosterone, thyroid disease, liver disease, or a testicular problem.
The article also says
A breast change by itself is rarely an emergency.
Every sign above is quoted from the sofidex article on Gynecomastia. If you are not sure, err upward — it is always reasonable to be checked.