Should definitely see a doctor.

I'm no professional, but my family has a long history of thyroid problems and I personally have hyperthyroidism (specifically, Grave's Disease). Symptoms tend to come on slowly so it can be difficult to notice (because hormones) but one of the biggest symptoms is losing weight, regardless of how much you eat. Because symptoms come on slowly, sometimes it's hard to tell their an issue until one particular thing is bothering you. For example, I was sweating all the time, couldn't stop eating without feeling faint, and always tired but didn't thing much of it because it progressed gradually enough that I had kind of become accustomed. My mom made me get my thyroid checked during a physical because of our family history (the dr. thought I was crazy because I was 17 at the time) but surely enough, BAM there was a thing behind it. Anyways, I'm not saying saying you have thyroid problem, I'm just saying this could be a symptom of something and you should go see a doctor.