Hamstring pain/spasm 4 months post ACL recon

I am 4 months post ACL Reconstruction (patellar graft) and I continue to have pain/spasms in my hamstrings when I try to flex my knee past 90 degrees.  This is especially true at night.  I do not have full flexion as of yet (lack around 15 degrees).   When I stretch or have someone else stretch me into flexion, my hamstrings contract and are very painful.  Any ideas on why this is happening.  I would think that my pain shoul dbe in the quad muscles.

 

Any advice is appreciated.