They gave me a vocal and written offer but still didn't confirmed my place

Hey, yesterday I ended up in clearing and found out real quick this place for comparative lit at Queen's Mary: I've talked to the head of faculty, did a little interview, and received an offer. After a while, I also had an e-mail from the same person telling me I had until six am to confirm my place both by putting it into track and send an acceptance letter to a certain address. I did both, but they still haven't confirmed my application! Is it possible they changed their mind and I should keep calling other unis or is this completely normal? Thanks in advance to anyone who can help me on this xx