I can't say for certain of course, but both my mom and brother were on Zoloft and lost their appetites...and nausea. It could be the drug, but best to check with your doc. If it doesn't go away maybe there is an alternative you can try.
Zoloft may cause loss of appetite in the beginning but the majority of people on Zoloft gain weight. However I agree, push yourself to eat healthy and add extra good calories if you are loosing weight.