Foul taste in mouth

I sometimes get a really foul taste in my mouth. Normally it happens when I'm eating. The taste reminds me of the smell of a rotting animal and the first few times this happened I thought the food I was eating was spoiled. This has happened several times. Twice I got the same taste in my mouth when I was not eating. I have only experienced a mild heartburn a couple of times in my life. I often get canker sores in my mouth but this has gone on for several years whereas the foul taste only began a few months ago.