What Causes Difficulty In Bending The Knee?

I woke up last night and couldn't unbend my left knee...I was sending the command, but my knee wasn't moving. So I straightened it with my hands and went to stand up and I collapsed. I layed there for awhile and then tried to bend my knee again and this time it worked. But now (the next morning) my left leg from the knee down feels weak...like I just ran a huge race...and like it could buckle at any moment and it feels slightly numb. No pain, though. And I'm 15 weeks pregnant. What do you think? Do I need to have it checked out.