Claims Ex-GF Kidnapped Their Daughter

NFL lineman DJ Fluker claims his ex-girlfriend kidnapped their daughter earlier this month -- and he's concerned things could get worse.

Here's the deal ... Fluker claims his ex, Kimberly Davis, was supposed to have a supervised visit with their daughter and Fluker's sister on Sept. 19th, while the Baltimore Ravens offensive lineman was away in Houston to play the Texans.

But Davis allegedly kicked Fluker's sister out of the apartment and refused to return to the child back to DJ. Fluker reportedly told Davis she did NOT have permission to keep their daughter overnight but Davis still wouldn't hand over the kid.

So, 29-year-old Fluker called police -- but Davis still refused to return the…