HELP!!! bottom of turtle has a dark soft rotting spot

I have a water turtle that is about 3in and noticed a few days ago it has a dark very soft spot on the bottom of his shell that looks like its rotting like fruit its a light black color and very, very soft. I took him out of cage and put him in a tub with paper towels and been feeding him worms with calcium d3 powder on them. What could it be from? They have uva/uvb light its the tube style light and a nice basking area that stays around 86-88 and waters at 70-72. And is it ok to keep him in tub with damp paper towel on one side and dry on the other? Any help will be appreciated. Thanks

I tried loading pic but looks like its not working I'll try again to load them. And the spot is a bit smaller them a dime. It feels like it will rip threw it I put a small bit of pressure on it.

Hi Scott, my name I Scott too! I found one of my little turtles today that has the same problem I think can you get pic? I need to find out whats wrong with mine asap. Thanks Scott