When you made $21,000 doing two jobs, each job withheld at a very low rate, possibly zero.
For example (to oversimplify): In 2012, a single person can earn $9,750 and owe zero tax. If you had a job that earned you less than $9,750 a year, no tax is withheld regardless of the number of allowances you put on your W-4. Then, if you had a second job with the same earnings of $9,750, no tax is withheld.
But, when you combine these two jobs, you now will owe tax on $9,750, which comes to just over $1,000.
To deal with this, use the worksheet on the back of Form W-4. It does a fair job of figuring out the additional withholding you should do in case of two jobs. You can also do a mock tax return and use the withholding tables available at IRS.gov, or get a tax professional to do this for you.