No heating and 23 degrees inside the apartment?

[Sonjing]

It is 4 degrees where I live, the heating is not on (central heating, LL has yet to turn it on for the whole building this year) and it is 23 degrees inside my flat. Toasty warm.

How is physically possible? Anyone who knows a bit about buildings and insulation? Is this even normal?

In all the places where I previously lived I would have been freezing with that temperature outside and no heating.

I am really confused 

Underneath me is ground. I don't have anything at the top.

Hard to explain, but my whole small apartment js like an independent extension built in between two buildings. So nothing underneath and nothing over either.