I didn’t experience pain but fluid can easily concentrate (a seroma - think
of the fluid that helps heal and protect a blister). It may certainly cause discomfort as it often exerts pressure and there may still be sensitivity from your mastectomy. Yes, the conventional wisdom is that it will disperse naturally, and medical staff are often reluctant to remove the fluid by aspiration as there is a risk of introducing infection. But the fluid can also linger and/or replenish. Ask your surgeon about massage to help the fluid disperse. A good lymphoedema therapist can do this. Ultimately you don’t want the fluid hanging around for too long, for a number of reasons, not least the continued discomfort. Best wishes.