18-year-old Whitney -- who was featured on the MTV show in 2009 -- tells us, she was living with her boyfriend and their son in an apartment in Georgia up until a couple of weeks ago -- but she decided to call it quits.

According to Whitney, she ended things because her man -- Weston Gosa -- "was gone all the time. He wouldn't come home until 3AM in the morning ... I was home with the baby and he was nowhere to be found."

But Whitney doesn't believe Weston was cheating -- telling us, "You know when someone is cheating on you. He was just out with his friends all the time getting messed up ... I don't want that around my kid."

Whitney claims the couple was fighting constantly and she just couldn't take it anymore, so she moved back in with her parents -- and took their son with her.