Pansy727,
You probably twisted your knee, be sure you are taking your pain medication as prescribed.
You are just a few weeks out, it does not take much to make your knee very unhappy, a quick stretch and twist will flare it up immediately.
Follow the Mantra, Rest, Ice, Elevate, and Medicate on schedule as prescribed. Watch for increasing heat, swelling, redness, and check for bruising later, it will take a while.
Call your OS in the morning and let them know what happened.
Keep us posted on how you are doing,