You can't build the 'lower pec' specifically. Some folks are genetically predisposed to fuller slabs, some have that knotted mass higher up on ther chest. If you have loose or fatty skin on the lower pecs, around or below the nipples it could be a case of gynochomastia which is what people mean when they say 'gyno' or 'bitch tits.'
If you're not juicing then you can get rid of this condition by reducing overall body fat, unless you've got some other hormonal issues. It can be a pain in the ass if you're one of those unfortunate souls who loose the tits last but for most guys it's the love handles and beer gut that are the problem.