Top differential - Infective carditis secondary to prosthetic valve, with concern about impact on cardiac function due to SOB on exertion and fatigue. Ddx to consider: Lymphoproliferative disorder HIV, TB or abscess, early menopause (primary ovarian failure) but there wouldn't be an actual fever & periods still present.
Great hx taking..It would be relevant to ask any lumps or bumps(lymphoma) , medication allergy (because of the long treatment duration) and any recent procedure e.g tooth extraction