Outside Knee Swelling Below Patella Directly After Exercise

Hi everyone, first post here:

I've been having some swelling issues on my knee, on and off, for the past 3 weeks or so. It started 3 weeks ago shortly after finishing a run, I had hopped over a rock and when I came down, I felt a slight pinch/pain in my right knee. When I got home, I went into a lunge and saw a ball-like swelling on the outside of my right knee, right underneath and slightly outside of my patella.

Since, then its gone down immediately when I ice it and doesn't come back when just walking or going up and downstairs. But, anytime i do any sort of kneed flexion exercise (squats, step ups, etc.) it comes right back in the same spot. One trainer at the gym told me it was "synovial fluid" build up, but he couldn't say what could be causing it.

Now, since the pain I felt on the run, theres been no further pain in the area where the swelling is or anywhere else on the knee. In fact, even while its swollen, I can still do squats and exercise with no pain, just a slightly tight feeling in my knee.

I've included some pictures of what it looks like after I finish working out (sorry for the poor quality/angles)

So my question: what is the most likely cause, what is best course of action? Is it most likely some sort of tear in a ligament, or just inflammation from poor stretching/mobility? I'm currently on an internship and won't be able to get back to my doctor until the end of the month, so I would like some advice on what it could be/ what to do in the meantime....I've stopped any lower body knee flexion exercises and running entirely until I figure out what's causing, because it's freaking me out a little :/ Any treatment advice, either from professionals or from those with similar experiences, would be hugely appreciated!!! :)
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