Some apples are naturally sour (green apples) I can't seem to find any info to indicate that sweet apples would become sour when they get rotten.

The apple should be crisp and not brown, if its too soft or there are extensive brown areas inside or out it may be rotten, if there are one or two soft spots and the rest is hard it is bruised, cut away the bruised parts

If you think it may be rotten your best bet would be to throw it away rather than risk it, better safe than sorry when it comes to the quality of your food

I'm no MD by any stretch, but IMHO its not likely to be a big deal, don't panic, if you have stomach pains that don't go away you might want to talk do a doctor, but I've eaten questionable fruit before without much consquence

i would say eat another