I slightly twisted my knee and heard a pop. there was no pain, no swelling. i could walk, put weight, although felt weak/unstable?

Probably OK: A serious knee injury almost always causes significant pain at the time of injury and when using the knee. If the knee still feels unstable after a week, see a doctor.

May need to see MD: Usually pops have to do with abnormal movement or tears of cartilage. In the knee this could involve ligaments, tendons or the meniscus. It is good that there is no pain or swelling but if your knee feels unstable then use an elastic bandage like a Ace wrap to improve stability and see your doctor if this does not improve in a few days.