Hello, I am relatively new at chess, but I am pretty good... I do, however, have a question about a rule I am unsure of.

I was playing against the computer (in ChessTitans on windows) and I was winning easily, so I decided to have a bit of fun. I got 3 of my pawns to the 8th file in order to redeem 3 extra queens. Once I finished this, I proceeded to get a checkmate on the computer's king. It showed I had a checkmate, but counted (and said) that the game was a draw.

Can someone please explain this rule to me so I am better able to win in the future.

-Ben