The water monitor is stressed. Leave it alone and make sure your husbandry is correct. Let it settle in, and you should see an improvement in appetite.

Here is a good site for monitor info....browse around:
http://www.proexotics.com/QUICKCARE_...TORS_1-08.html

That 20 gallon tank is too small for a foot long reptile.....get it into something bigger. If the water is taken care of properly, it could easily get over 6 ft. long in a years time. You will need a room-sized enclosure to house an adult water monitor properly.

....and crickets don't sting.....they do bite though.

The problem is the conditions you are providing. Provide for the animal properly and you should have nothing to be concerned over......

...and I would suggest researching care BEFORE you actually purchase the animal next time.