Is it common for cyst to form under nipple in male? swells up oneday , next day smaller. should i be worried about male breast cancer? 27 yr old male

No and no: Cyst in breast is no common in males. Breast cancer is rare at your age. But something is going on. Go see your doctor or surgoen.

Breast cancer: Breast cancer is rare in males. This should be evaluated by a breast surgeon so that the appropriate study like an ultrasound can be used to help determine the next steps.

Not cancer: It would be highly unlikely to be breast cancer, but it could be gynecomastia, where breast tissue develops in men. It could be related to meds or other causes and your doctor can review these with you. Or it could be either a sebaceous or epidermal cyst, which can be managed quite easily.