You find that the more of your girlfriend's milk you drink, the more calm you become, almost as if you are being put into a trance. You didn't even notice that your teeth had all disappeared, nor that your body adjusted to having a mostly liquid based diet, which focused almost entirely around you consuming either breast milk or cum. And while other fluids were edible still, none of them are nutritionally better for you to consume than those two.
While you kept suckling your Girlfriend's nipple, she was slowly slipping a finger inside of your new pussy. You can feel the finger enter you, and enjoy the feeling it brings you greatly. However you never stop drinking, not even to moan. You can feel her picking up speed as she rubs and fingers your pussy faster and harder. You would be squealing with pleasure if not for the irresistible urge to keep on suckling more and more milk.
As this continues, you slowly begin to age up, until you are around 19/20 years old. Still completely toothless, but your breasts have swelled significantly. What were once a manageable pair of B cups, now are pushing to be F cups. And on top of that, they too are lactating. No where near as much as your girlfriend's, but more so than they should normally be. Watching your boobs grow and fill with milk had caused you to have a few orgasms as well.
It was a minute after you sropped aging that suddenly your seal over your girlfriend's nipple that your lips were making became very strong. Your girlfriend also noticed this and looked down to investigate. What she found out, and then told you about, was that your lips had merged together, forming a perfect ring.
Conveniently, she had just ran dry, so you let go. What slides out of your mouth is not the normal nipple you were expecting, but a 5 inch long, 1 inch diameter, teat that looked as if it belonged on a cow's udder. You look at her other breast to find a matching teat. Your girlfriend lifts your head and slips her tongue into your wet lip ring of a mouth. She then picks you up and removes her clothes fully, revealing the dick you had figured out she had grown a while ago.
Releasing her tongue, you lowered your drooling orifice down onto it, instantly sealing it inside tightly with your lips. You drop to your knees and begin moving your head back and forth, pushing your lip ring across the full length of her shaft, and then pulling it back to just before the head. Back and forth, you continue to suck her cock as she simply stands in front of you and holds your head.
"Wow your mouth feels amazing. It's like it was built for doing this specifically." Comments your girlfriend. But little did she realize just how right she was.
Your entire mouth structure had become optimised for sucking liquid out of things, often by using pleasure as a means for extraction. The walls of your mouth were on par with the best of pussies. You could use them to gently stroke whatever enters your mouth hole, literally milking it for everything it has to offer. Your tongue long since has changed into a thin membrane near the back of your mouth which you use to control the strength of suction that the vacuum seal can have. It also prevents you from swallowing early. And only once you have pulled as much as you can fit inside your mouth out, does it let you swallow. And while this membrane exists, it doesn't prevent you from speaking, or deep deepthroating a dick as it can be moved out of the way at your will.
You mouth was a perfect hole to suck cum out of a dick and milk out of a teat. And lucky for you, your girlfriend has both, as well as a body that not only produces rapidly, but in excess as well.