I flagged but I thought it would be called a draw. I lost???

What is the rule for calling a draw after time runs out?

Thanks

You lost because there is a legal series of moves that lead to checkmate for black. Without the white rook and pawn, it would be draw

Ok Thanks.

[Variant "From Position"]
[FEN "8/7k/6n1/1R6/6KP/8/8/8 w - - 0 1"]

1. Rb2 Kh6 2. Kg3 Kh5 3. Kg2 Kxh4 4. Kh1 Nf4
5. Rg2 Nd3 6. Rg8 Kh3 7. Rg1 Nf2#

Here's an example of how black can win in 7 moves.