I'll second seeing a doctor, but as counter intuitive as it may seem you need to eat more just as others have already suggested. As long as the calories are healthy you'll find yourself in a better place in short order. Also, at least for a few weeks try tracking everything you eat. Often the nibble here or there add up during the day. There are numerous online trackers that can help with this. That written, I won't lie. At the beginning it can be a PIA, but if you can gain some knowledge then it's worth it.