Why has my mother become more hungry than she usually is?

My mother is 94 and lives with my husband and me. I'm her full time caregiver. I love to cook so she enjoys healthy meals every day. Lately, she's been asking for more to eat. I'm now providing additional food every 2 hours or so. Can anyone suggest why she is hungry more often than usual? She has a balanced diet and also has fruit, nuts, cheese, small protein, or soup between her regular meals. She only gets healthy snacks, no junk or processed foods. I'm stumped!