update
Thank you all guys for your replies. I successfully transferred that domain out of NS, here is how.
First, I was able to obtain a $8 credit from GD to spend for another order.
After checking the whois data once again at NS, I noticed that my old whois was back.
The email address was my old one instead of pendingrenewalordeletion. So I quickly decided to request a new transfer to another registrar: I succesfully received every email to make the transfer.
Now I have the domain transferred out on NS, but I really don't understand what happened exaclty. Why they initially changed the whois data, just after 2 days from the domain's expiry date? And why they changed the whois data back to the original?
Nothing is clear to me at this point. However I just wanted to share this with you, thanks again.