I have trouble bending my knees or keeping them bent. What does this mean and what should I do about it?

Knee effusion likely. Knees that are swollen with fluid (effusion) are more difficult to bend since you cannot compress fluid contained within the closed space of your knee. A bent knee decreases the overall volume of the knee joint, whereas a partially bent knee provides the most space (volume) for fluid to collect. If this is true, you likely cannot fully straighten your knees as well. Seek thorough knee examination.