Are lumps behind the nipples a symptom of gynecomastia or just normal hormone fluctuations?

Neither. Lumps or masses beneath nipples may be concerning and should be examined by a breast surgeon. Gynecomastia is typified by enlargement of the entire breast, not only certain nodules beneath the nipples.
Needs Evaluation. A mass behind the nipple/areola complexes should be evaluated. This could be gynecomastia or breast cancer.
Yes. Could be either. Would have a surgeon incheck it especially if any history of breast cancer in your family.