Rheumatoid Arthritis - Seronegative?

I was diagnosed with RA on December 3rd. My ANA was borderline, and I thought my doctor said that RF was positive.  I had an MRI of right hand and wrist to finalize my diagnosis.  It came back with some carpel erosion. 

Today, I started MTX 7.5mg and will build up every week until I reach 20mg I was told.  I am currently on 5mg prednisone.  (Main reason for prednisone is for pain because I am allergic to all anti-inflammatories.)  Also, taking 1mg folic acid daily.

Now, my question.  The doctor told me today that my last two blood tests were seronegative.  As I researched this on the net, I found that this is a negative RF test.  So could he be wrong about my diagnosis?

Some quick history:  I started with pain in my neck and wrists in July and it gradually got worse and could hardly get out of bed and walk by November.  (Pain seemed to be in all muscles, too.)  But, I would have my good and bad days.  Pain was really bad and that is what caused me to go to the doctors.  Went to Several Doctors until I got in with Rheumy.  He prescribed medrol pak for 6 days.  Pain went away ... this was wonderful!  However, 3 days off of pak and pain was back.  So he started me on the prednisone and here we are today.

What other factors or blood work or tests indicate RA besides an RF test?  I read on net that doctors just diagnosis RA even with a negative RF because they don't know what is going on.  Is this true?  I am 42 years old and have been an athlete most of my life and still am.  Take care of myself the best I can.  Now I am very confused after today's visit.