Severe knee pain only on flexion. What am I possibly dealing with?

Internal knee. It could be many things but consider internal knee derangement issues like cartilage, joint mice, ligaments, tendons. Does the knee lock? This can be a clue. Even though you are getting knee pain also consider structures around the knee like bursas. Your evaluation will include an examination, imaging like x-rays, ultrasound or mri. Therapy includes exercise, surgery depending on findings.
Knee Pain. It would depend on where your pain is located. On the side of the knee, possibly the meniscus. In the front, could be a knee cap problem or tendinitis.