Car smells of 'burning and fuel'

I was overtaking a friend today and he said my car smelt of the above. I'm a bit worried about it! However, it's a really hot day and I had been sitting in traffic for 20 mins in a low gear (enjoying the sound a bit too much!). Would this be what caused the smell, or could it be something more serious? I've never smelt it previously when revving the car stationary.
Cheers