Just bought a phone and trying to con me!!!

Hi, Just after a bit of advice. Just bought a Galaxy S4 off somebody locally. I saw it listed on Ebay and asked her if she would end the auction and sell it me cash on collection. Paid £200 cash and on the way home she text me saying "bring the phone back or my money as you have not paid for it". I text back saying "what the hell are you on about?". She text back saying "you never paid me". I text back saying "dont even think of trying this with me , you had the money and signed the receipt , i had the phone and went" Now... when i got there i almost turned away as her house looked really rough and when she opened the door , it was obvious she was a bit of dirty and im sure of it, into drugs, and don't get me started on the smell. I had her signing a receipt saying it has been sold for £200 and it wont get barred. My question is, firstly , why would somebody do that? Is it to fuel her argument that its been stolen when she tries to bar it? Secondly, im not going to get this taken off me am i as i have called the police to advise them of whats gone on and they have an appointment for me at 6pm tonight. Any help really appreciated Matt