If you think there's a chance you might be pregnant, take a home pregnancy test. I'm sure you are if you only eat one meal a day and are still putting on weight. Plus, like vmvnpv said, you aren't getting the nutrition you need.
You are not getting the nutrition you need. Therefore when you eat one meal your body, which is starving, is putting the nutrition and food you are getting into fat storage because it doesn't know when the next time you are going to eat will be. Chances are you are also overeating in that one meal because you are so hungry. The best way to lose weight is to exercise more, eat smaller portioned meals (but more meals) in a day and make sure they are healthy meals (i.e. LOTS of vegetables, a protein, etc). Also, cut back on the coffee and drink more water. Your body is getting dehydrated and all that caffeine isn't good for you.
when was your last period? I'd take a home pregnancy test first if you're late for your period, although lots of things can cause you to gain weight.