I want to sue the payroll! need help... Can I sue?

I really need help with this!
I was working for Estee Lauder full time..
weekly I called my hour's in and I always's did get paid on time.. But one day payroll called me and told me that they over paid me $6,200 and I had 2 weeks to pay them back the money or they was going to send the Cop's to my home door! I really didn't know what to think about all this because they made me sign a latter saying that I know that I was over paid from the payroll.. I have been crying day by day because they have put me soo down! They told the co-workers and other people that I was calling in hour's without working and that's not true! How can I call in more then 400 hour's and payroll send me the check's? I think before they send me my money the job of the payroll is to look at the hour's I put in and my boss---Then pay me?
The other thing is that I paid them the full money in money order's for $6,200 and I paid it on the 1st week of march 2008... I call that 1800 number in back of the money order and it say that they have not cash all the money order yet! I think something is going on and I really don't know what it is!

Also today when I went to get my mail my words was OMG! because they sent me a refund check for the amount of: $565! I don't understand why they sent me that refund without they calling me or sendind me a letter saying why they are sending me this refund? When they made me pay $6,200 back to payroll! They put me so lo