Thank you for your question. Shrinking cleavage can be due to a number of possible causes including weight loss, hormonal changes, and tissue sagging. A photograph may better demonstrate what is causing your breasts to shrink, but if you are aiming to restore volume, see an ASPS board certified plastic surgeon for consultation for possible implant placement.
You must be losing weight for your breasts to be decreasing in size at the age of 22. Options areput on weightbreast implantfat transfer