Yes please talk to your dr, there are many things that could cause rapid or excessive weight loss and some of them can be serious. In addition to Thyroid and GI testing maybe ask about diabetes. It seems odd, but it can cause weight loss.
I would recommend going to your doctor so he or she can run some tests. One of those tests should be a full thyroid panel--TSH, Free T3, Free T4, and thyroid antibodies. Also ask your doctor to send you to a G.I.-- I sounds like for some reason or another you are not absorbing your food.
I hope this helps,
achilles2