Echo what the above 2 posters said in regards to exercise.
Very, very well done in losing all that weight and changing your life for the better!
If you want to get rid of the bump 3 things have to happen in training (notice I assume nutrition is already important, only talking about training here):
1) you need muscle. You can't flex bone, and even if you're very low in body fat abs will be hard to come by with no muscle to show through skin. You don't have extra fat now as much as you have extra skin and no muscle. You need enough muscle to show underneath the skin in order for you to look as lean as you are. This also interfaces with nutrition because you can't grow muscle starving yourself. That doesn't mean bulk, that just means maintain or slightly and slowly grow. Slower than "usual" advice.
2) you need to work your abs (obviously). A six pack isn't just about doing ab work, but it does help. Alternate heavy and light days, with light days being higher rep on whatever you're doing
3) you need to train your glutes (weird advice i know). One thing aside from fat level and muscle level that determines that little pooch in the lower ab area is how your hip is tilting. If it is level that little extra bit is minimized. If the front of your hip is tilted down and lower than the back of your hip, it can weirdly make your stomach appear bigger than it actually is.