Is it possible that I'm not eating enough and that's why I'm getting sick?

I've been really nauseous and haven't been eating breakfast or lunch. I drink coffee in the morning, water throughout the day, and then whatever my roommate cooks for dinner (Which is usually like protein and veggies// basic dinners). I fainted today after taking my dogs for a walk. I'm in pretty good shape so I have no idea why it was so hard for me to walk them. My roommate said he thinks it's because I'm not eating enough. But I feel sick when I eat!