Debt verification - help

I have a question. I have a company called PD6/Express Cash. A lady, Sharon Williams (800-859-6439x248) is emailing me and calling me. I had spoken with her months ago and I have paid them off $390 like I agreed with her. Now she is calling and emailing me stating I still owe $390 in fees. I faxed her a debt validation form along with a cease and desist letter requesting proof of my original signed loan along with request for where these fees are coming from. They must have record, right? So she just called my 4 times with 4 messages stating she received my fax and has reviewed this with others and they are aloud to call me because I gave that information on the loan. Also, she says I am aware of the loan, which I was, but had thought it was taken care of back in October. So I'm wondering if this lady actually works for the loan company or is she a debt collector. She said the only way she can get a copy of the signed loan and the other information is from a lawyer and it has to be a sopeona. My question is, does the original collector have to acknowledge a cease and desist letter and also prove debt verification? I'm confused on what to do. I don't really want to call this lady back because she is going to give me the run around. I plan on mailing the letter through certified mail also so I have record. Please someone give me insight on what I should do next. Also, I need to figure out if she works with the original company or another debt collection company. I have worked with her before and had no problems paying $390 before but now I can't pay another $390 in fees when I thought we already had this taken care of back in October.