Dark circles i have heard are generally related to lowered kidney and liver functions. Fast food is known to damage the liver with the trans fats and other chemicals etc.Liver,colon and kidney cleansing will definitely help.check out other forums on this site.
I too am very thin but when you feel well and happy it doesn't matter really what the figures on the scale read.
all the very best and the changes you are making will definitely benefit you in the long run.