Must I make up for the days I broke my fast in Ramadan due to masturbation, or is feeding a poor person sufficient, given that I suffer from irritable bowel syndrome and feel extreme fatigue when fasting?

All days intentionally missed must be made up. Feeding the poor is not a substitute as long as you are able to make up the fast. In addition to making up the fast, you must feed one poor person for each day, due to delaying the makeup fast until the next Ramadan (definition) began. If you do not know the number of days, then make up what you are certain or highly likely will absolve your responsibility. You must also repent to Allah for committing a grave sin.