You might!  Effectiveness and efficiency is one of the best things about MFR.  It can make profound and dramatic changes in a very short period of time.  Sometimes however, you will feel sore after a session, it may feel like things have gotten worse or a flare up is occurring.  This can feel scary and may shake you up.  This is good!  It means the treatment is working and the tissues are being released.  This should go away after a few days, and you should feel much better than you did before the session.  Be sure to drink lots of water and take care of yourself, and to discuss it with your therapist.