Severe knee pain only on flexion. what am i possibly dealing with?

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.

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.