I have pain in my left knee, is seems to be mostly under my kneecap, more on the left side. i feel pain when i need to get up after sitting, or just before stretching my knee completely (at 20 degrees), and when i twist foot while having knee bent?

I assume there's been no recent left knee injury. However overuse syndrome from exercise,work or chores is a possibility. Resting the knee should resolve this. If your knee symptoms appear persistent, get checked by your MD for tendinitis or a condition called chondromalacia which occurs behind the kneecap & is common in young women as they tend to have more pressure on the sides of the patella

Very common for women because of biomechanics of knee function and femeral anteversion ( knock kneed). And having wider hips then men. A podiatrist can provide a custom orthotic to allow proper function of knee cap. Patella strengthening the quadriceps muscles also helpful