Are you still seeing a physical therapist? One good way to work on flexion at home is to sit in your desk chair (if it has wheels on it), place your affected leg out in front of you, with your foot flat on the floor and then slowly scoot/roll the chair forward, keeping your left foot planted the whole time. This will cause your knee to gently bend. Start off slow, you don't have to cause yourself to be in pain but increase the flexion little by little each time you do it! Best of luck!!
As long as you keep going with your exercises regularly, the movement should come back. Recovery is rarely all progress, it goes up and down. If there is any swelling that will limit movement. You can help reduce that with elevation, ice and movement. You may have just overdone things slightly which has caused it to stiffen up.
Try and make sure you are exercising your knee regularly during the day. Have a look at http://www.knee-pain-explained.com/knee-replacement-exercise.html for exercises that may help. Hope it gets better soon!