Mild knee pain when bent. can't bend more than 90 degrees without assistance. walk and bear weight fine.

Have it evaluated: If the issue gives you enough aggravation, see an orthopedist and have the issue evaluated.

Could be torn meniscus: You should see an orthopedic surgeon, you might need an mri.

Hard to tell without: More history or physical exam. It could be anything from a mild ligament strain (which it sounds like) to a meniscal injury. Also, if in the back of your knee, you feel some fullness, you may have a bakers cyst, which is very common and can be easily drained if it is too much of a pain issue for you. I wish i could be more specific, but with little history that's a couple things that it could be.