I Talk Down To My Wife -help

This morning my wife said "We don't have any compost". My reply was "How do you know? We have a bag in the store."
It's not what I said but how I said it. It's not the first time and apparently I've been getting worse.
She was upset but I hadn't realised how bad it sounded and carried on in the garden. After a few minutes I went in and she was crying.
I feel so bad about it and need advise.
I'm from a military background and have been brought up to tell the truth. So making a statement when not knowing that it is true goes against the grain.
I could have just said - "We have a bag in the store" and that wouldn't have upset her. So why did I do it?
Help.
looselowerlip looselowerlip
51-55, M
May 25, 2012