I agree with estherderu.
Your lumbar lordosis could be absolutely "normal" and its current prominence might have been caused by weak muscles or ligaments.
Have you changed jobs. physical activities or eating habits?
A straight back is pathological. If you want to avoid your tummy sticking out, train your core.
And sorry guys, as much as I like to promote our profession, this guy does not have a pathological health problem or pain - he says it's just visual. Muscle training can be done by any professional working in this area, e.g. personal trainer, exercise physiologist, Pilates etc..
Cheers,
Fyzzio