Posterior-lateral knee pain behind left knee

After some searching to identify where and what the pain is, I think it might be posterior lateral pain, but I can describe the location because I could be so wrong! The pain is behind my left knee, slightly toward the outer side (I’d call it the outside back corner of my knee) and directly on the tendon area that flexes when I bend my knee.

It hurts most with bending the knee, during and immediately after a work out and the area is tender when pressed. I can also feel a small firm-ish bump there, when it is flared up! Aside from that bump, there are no other physical characteristics (no swelling or bruising or redness).

I did take some time off after my first marathon in November, (where it would flare up occasionally after hard runs); I am back to running pretty consistently again, and have noticed it during and after most runs now (including a hilly half in April).

 I know it is an issue and not sustainable, so I am looking for some guidance on what this pain can be, how to treat it, and what I should be doing now to ensure it doesn’t get worse later!

Thank you!