I don't know much, but I'll see what I can say.
I believe this pain and tightness is a direct result of me listening to the wonderful advice on not collapsing my knees inward on the upward motion of my squat. I'm not being sarcastic here not at all.
I think there is a connection. Practically when your knees go inward during squat, you are internally rotating your hips. Only a little, but still. And the tensor fascia latae actually does the inner rotation of the hip/thigh when your hip is flexed, aka when you are in the down part of your squatting. So I think you worked that muscle much more before, and it could be possible that it is now tense and thight. When you stopped doing it, there might be sartorius or quads pressing the tensor fascia latae, or something like that. Take those with a grain of salt, they are just quick theories.
Or you may overwork your sartorius, which does the external rotation of the hip. So you're now fighting for it to not happen, which activates the sartorius much more, thus causing the pain/swelling. There is a possibility the sartorius was more inactive before, when you did squats with a slight internal rotation. How bad is the swelling by the way?
Kpj:s video also shows a good stretch for the whole quadriceps muscle. When you extend the hip straight or a bit over when flexing your knee, you're also stretching the long part of the quadriceps muscle, (rectus femoris), which is the only part of quads that go over two joints (it's origo is in the ilium)