It is perfectly normal. There is nothing inherently wrong with a relaxed abdomen "sticking out" like that. This is how a human beings look like when their body is in rest-and-digest mode. Even the abdomen of strong, perfectly healthy people can stick out, when they relax completely.
That does not mean you should spend the whole day like that of course. If you have an active lifestyle, a balanced exercise program, and your diet is under control, there is no reason to obsess over your looks. In fact, misguided advice to always tense and pull in your abdomen can be bad for your digestion and can cause unnecessary stress.
What is really causing your stomach to stick out on the second picture is called an anterior pelvic tilt. If it becomes chronic, it might cause back problems, and require fixing (preferably by a physio). But again, it might not be a chronic problem for you right now. If you want to read more about it, take a look at this answer.