I have a viral upper respiratory infection and in the past 3 days I've lost 12 pounds and I have been eating full meals do you know what it could be?

Highly unlikely. See your doctor if it is not simple dehydration. Start drinking as much water as you can at least 60 ounces a day. If this does not help you check with your doctor. Also if your doctor diagnosed your condition you need a follow up. Only obese people are able to lose that much weight relatively quickly from not eating when sick.