How cone when I smell or think about food, I get nauseated?

This just started recently. everytime I think about food, especially fast food, I feel very nauseated. Also when I smell food from restaurants, fast food places, just things cooking in general. I get nauseated. I lose my appetite, which really quite sucks because I use to enjoy food. Now everytime I eat, I have to force it down. I've never actually gotten sick but I've come quite close. Is this just a mental thing or could something be physically wrong with me? And I couldn't be pregnant, I'm on my menstrual cycle at the moment and it's regular.