Seeking diagnosis and therapy for minor but puzzling knee problem

On Friday I was doing my exercise program, as proposed to me by my personal trainer, which included 10 “Chair Squats” and 10 each leg “VMO Single Leg Mini-Squat” plus other exercises. After one set of my exercises, but not as an acute injury onset, my right knee suddenly felt like an electric storm or lightning storm in the front of the knee when I moved my leg. I could barely walk for the pain. My hypothesis is the trigger was those specific exercises mentioned. I tried for perfect form on these Chair Squats, rising with weight on my heels and not forefoot, and leaning my torso forward at moment of standing up from seating so the uplift is almost entirely straight vertical. I heard slight snapping from both knees as I did each of the 10 Chair Squats, but no pain during or immediately afterwards. The pain when it came on felt to me more like an ‘itis’ (as in tendinitis) than a broad muscle soreness, though I am known for tighter than average muscles, so it could be due to tight or imbalanced muscles pulling on the knee.

The longer history is prior to Sumo Squats I did deep squats from standing almost with butt touching heels, but would get knee snapping on rising if my weight was not solely on my heels. Several big snaps would lead to knee soreness later (though not of this electrical storm kind). I then tried Sumo Squats but again got knee snapping if I bent too low and didn't rise with weight on heels. Also, I had been diagnosed with “patellofemoral syndrome” in the past. In the past, I would get soreness just below the right knee cap in the groove between the patella and lower leg bone in front. The idea of the VMO Single Leg Mini-Squats is to strengthen the VMO and thereby strengthen my knee to hopefully reduce the patellofemoral syndrome and reduce the snapping.

On Friday I could barely walk for the rest of the day, though it seemed to begin easing later in the day. Saturday it was easing but still some movements triggered the soreness. By Sunday, it seemed 95% back to normal. Overall, it seemed like peak soreness at the start and a linear rate of easing/heeling over 2.5 days. This would be a minor knee problem compared to the major knee problems that some people get, but nevertheless it is annoying.

What I’m looking for is thoughts on diagnosis, and recommendations for therapy to prevent aggravating the knee again. Is the knee snapping the key sign, which must be prevented? Can I only do squats, if I can avoid any snapping at all? That would mean going back to Sumo Squats but doing them moving butt strictly backwards and not down at all, as that seems to prevent snapping, though it's a fine line to get the Sumo Squat right and I often do it wrong and still get slight knee snapping. Is there a way to prevent my knee problems, or should I just live with it by avoiding anything that aggravates it (i.e., avoid anything that might cause knee snapping).

Big picture: 60 year old male, otherwise healthy, FitBit shows I get many days per week of 10,000+ steps.

Thanks.

Update Tuesday: My personal trainer felt my knee and suspects it is tight VMO, perhaps from the VMO strengthening exercises. I also should clarify that what I mean by "snapping" is an audible, sudden snapping sound. It is not a crunching sound. Both my personal trainer and I are puzzled as to what to do next with regards to my fitness program and prevention. Maybe I should just follow a general knee stretching and strengthening program, and not worry about specific diagnosis? (Any general knee stretching and strengthening program you would recommend?)

Frank