It will help if you eat some protein with each meal. Keep the simple sugars low, but boost the protein levels and complex carbs. Watch the fats as anyone would. Are you eating enough? Usually you get your best meal in before you start your day and/or at night when the Ritalin wears off. Three small meals sounds like you are possibly dieting and you may need more food, at least for now. You might consider taking an over-the-counter product for upset stomach with your Ritalin dose once a day if nothing else works.
Are you allowing yourself enough time for sleep? I ask because you mention taking Ritalin after you come home and it sounds like in the evening. This could prevent you from getting enough sleep. Sleep is essential for repairing your body from the day's demands and not being totally drained in the afternoon or at night. Look over these basics of meals and rest and see if you have room to adjust them in your favor.
Is there any other stressor in your life right now? My kids used to have hours of hard play after their Ritalin or current drug wore off. My adult son not so much. Write to me by private email on this site if you have another question or want me to ask my adult son with ADHD something.