Not a pro..... so take with a grain of salt.
High chance probably runner's knee/jumpers knee. Gets better as the tissue/patellar joint gets warmed up, but after that cool down and sesssion over, inflammation ramps up again often even lagi worse than before.
Do, rinse and repeat........ terok to the training progress as it never heals and so you scared and so you are forever stuck at a certain (lowish) fitness level etc.
Can try a multitude of solutions, cut training load (vol and/or intensity), let the inflammation go down first......it will be up there first as it tries to cope and inflammation is essential as part of the recovery process. But sometimes if it interferes with your daily activities/work....then it is essential to cut the inflammation to a more manageable level with NSAIDs, ice etc.
My belief is those knee guards , glucosamine/chondritin/MSM wont do much. Neither would denatured type II collagen (UC-II). Not the root cause.
If not, it could be some wierd biomechanics that is causing issues with your gait and rubbing the joint a bit due to misalignment, mistracking a bit here and there. This one i dunno the detailed science..... i just read and regurgitate as one of the possibilities.
Could also be some muscle imbalance, like hamstring weak (correspondingly overly strong quads). Anterior tibs, gluteus medius / or even the partner muscle TFL.....possibly one of the main causes is some muscles that is located away from the knee (not the calves/quads). This one you need self try some strengthening, if it gets worse and is chronic then gotta see sports doc liao.
If it really does improve and you solve it after a period of few weeks or even 2-3 months, then you might really wanna restart by really slowly progressing it up....esp never overdo 1 or 2 single sessions of jumping big in volume and/or intensity. And add in good strengthening. Can try plyometrics as well. Employ slow progressive overload for strengthening/plyo.
I tell you 1 story, not running. I tried 1 serious session of one arm deadhang. Ego deadhang lah. Really whacked it hard, like the no pain no gain method. Just hung on for dear life thinking "this is good training to failure or past failure), and my arm twisted around a bit without control (and legs forgot to try stabilise by stabilising at the side columns of the pull up bar).
My tendons/ligaments forearm nearer to the wrist pained for like 3-4 weeks total. 1 single ego session was all it took. I mean, i train to failure many times but it is a fine line between ego and failure/drop sets.
I know it was that 1 single session coz i tried it with my right arm and i also trained with my left arm but i didnt cheonged the left one.
ps. I just finished my legs strengthening, and yeah i just do everything including the small TFL and hamstring, anterior tibs, psoas, calves. Just do as isolation exercises, no harm one. In addition to the large compound stuff for brute strength, bulgarian split squats, single leg deadlift or single leg RDLs.
Special mention for weighted walking lunges as well, really very good.....just that for me i use bands and i dont have a set of dumbells/kettlebells in order to progress to say 1/3 to 1/2 bodyweight total. Also it is by default a unilateral exercise, some pax might not like single leg squats/deadlifts