I received a w2 after I filed my taxes. it added $508 to my income. im in the 12% bracket and my taxes went from a $343 overpayment to a $114 underpayment. any ideas why?

Ignore the Refund-O-Meter  and look at the actual 1040X ...  what changed in column B?  Did you lose a credit ?

Column A should have the figures from the original return, Column C the corrected figures and Column B the differences between the other 2 which needs an explanation on page 2 of the form 1040X.

If you completed it correctly you will see an entry on either line 16 OR 18:

On line 16 should be the amount you paid with your original return. 

On line 18 should be the amount of your original refund you received.

Then you will see your extra refund on line 22 OR  the new balance due you need to pay on line 20.

And  do NOT amend the return before you have received the original refund !!!

When you did an Amended return?  You will still get the full first refund.  Then the amended return is only for the difference you owe back.  So really your refund should have only been $229.