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If it's fat, you need to lose fat. Which means eating healthy and breaking a good sweat.

If it's not fat....it's probably skin. And it's only so elastic sometimes, heh.

yes what kemma said! Think about tucking your tailbone under you slightly when you stand and drawing in your lower abs. Envision a string pulling you up from the top of your head.
Your lower abs (well, the abs in general) are shot after carrying baby. All of the above nutrition stuff is right on target, if its belly fat, which is where some people just carry junk, then diet/ exercise will work on that. But that poochy belly can also be your lower abdominals not firing during normal every day activity. Google "transverse abdominal exercises" for some good ab ideas. Its good to think of this as sort of a multi faceted thing: diet, exercise, abdominal strength/posture
however first make sure you don't have diastisis recti. many ab exercises can actually make it worse if you have a separation.

also, with regards to the jeans fitting. Your pelvis may be wider than before. It certainly narrows after birth, however the upper bones sometimes remain wider than before.

(i'm a PT, btw)