Is practicing masturbation after a three-month cessation considered unlawful (haram), and is the yellow fluid discharged after it a symptom of illness or a natural occurrence?

The act of masturbation is forbidden, and it is obligatory to repent from it. If the fluid that exits comes out in spurts with pleasure, then it is mani (seminal fluid), which necessitates ghusl (ritual bath), even if its color has changed. Al-Nawawi mentioned the characteristics of mani and its changes while still necessitating ghusl, such as becoming thin or yellow due to illness, or exiting without lust, or becoming reddish due to frequent intercourse. It may even exit as blood and still be pure and necessitate ghusl. You can consult the counseling section regarding what you mentioned about your illness.