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.