Knee Pain from running

Hello guys! I am Johnny, I am 240lbs and 6’7 tall. I have a background of strength training so I am not new to it. But recently, because I am a “gym-bro” haha, I neglected the leg training (I know, a stupid decission). Probably did not train them in the last 3 months. One month ago I started running after a loooong period of time so I can prepare for law enforcement. 5 days ago while I was running, after approx. 400m I felt a very sharp pain just below my left kneecap and then it started radiating to the back of my knee. I stopped and I tried to walk, but I was not able to bear weight on it and it gave me the sensation of “soft leg” like it was made out of rubber haha. There was not swelling at all (at least I didn’t notice). After that, day by day, the knee got bettet until yesterday, when while I was getting home I began experiencing the same type of sharp pain and now it remained the same. I noticed that if I stand or sit for too long, when I try to walk is painful for a couple of steps 8-9/10 pain (hurts like hell just below my knee cap and behind the knee). After some steps, the pain subsides to a mild pain. I saw on my fitness app that I increased my step count from 3k to 5,5k that day. I notice some tender spots when I press 4-5 cm below my knee cap. I also noticed that it does not bother me too much when I climb stairs or when I go downstairs. I am familiar with your content on pain and today I went to the gym and trained. I did not notice any pain while squatting, maybe on the last set ,@8 RPE, a very mild pain 1-2/10. I was able to squat 225lbs for 5 @8. After that I tried to do some unilateral work, knee extensions. I did 3 sets of 12 with a 3-0-3 tempo with just one piece from the entire stack (I don’t know how that piece is called) and I did not notice any pain at all. Then I tried to lift 2 pieces from the stack, but my knee started to hurt below my kneecap so I stopped. Then I went to leg curls and with my left knee I was not able to even lift for one rep the first piece from the stack.

How should I manage this? I found that the squat is my entry point in the “rehab” process and knee extensions too, but I have to load these a bit slower. When I get up from the bed or from a chair where I stayed for more than 10 minutes, it hurts very bad, I can’t bear weight on it until I do a couple of steps and the pain subsides. I know that biological changes are not linked to symptoms, but I fear that it may be a PCL tear or a meniscus tear( I know the diagnosis do not change the approach of rehab too much). I also think that it may be a Patellar Tendinopathy, but I know the approach would be the same for all of them.
I noticed that if I stand and try to flex my knee, I notice a painful stretch below my knee cap, and I suspect that it might be the Patellar Tendon.
Thank you so much for your time and for your answers!

Anybody, please?