I am not sue how she can take half your paycheck. I assume that some documentation was sent to your employer to withhold income and your employer decided to follow through.
You need to find out what documentation was given/sent to your employer to commence this action. You will then need to determine if such notice was proper or not based pursuant to the Notice to Withhold Income for Support Act. Read state law 750 ILCS 28 or seek a free consultation with an attorney.