I am so sorry you are going through this. Are you sure it is AF? I bled red (sorry TMI) through my 6th week of pregnancy with DS and he is now 2 years old. I hope you get some answers soon.
This exact thing happened to me with my first pregnancy...
I was in beta hell for 2-3 weeks......the doctors feared ectopic and wanted me to take the shot (my beta never got over 1,500) but it kept slowly rising just like yours....I eventually started to bleed and lost the pregnancy at 7.5 weeks...
I am sorry you are going thru this.... for a happy ending.....you never know, miracles do happen......