I assume you mean 2lbs., correct?
Yorkies only weigh 6-7 lbs to begin with...
Sudden intake of more water, excessive hunger, etc. are symptoms of various diseases such as Thyroid Disease, Kidney disease, Cushings, Diabetes, Liver disease, along w/others.....
All of these are treatable & easier to control if caught early......Please don't wait......Get your guy in to be checked ASAP. Good luck and let us know what you find out......