Knee Problem - stiffness/catching/slight pain

I had a popping/knee partially giving way last Thursday, but it seemed to rectify itself and I was able to walk. This remained the case until Monday when I started to feel tightness in r/knee although no loss of movement. Some slight pain below kneecap into calf. It is causing me to limp. I have no pain if resting but when walking, it feels tight/stiff and like it will give way. Today, it feels like something is catching inside my knee. I am using ice/resting it, but it seems to be getting worse not better. I have made an appointment with a physio for Wednesday, but grateful for any advice as to what I should do before then. Although I am not in a lot of pain, my knee feels very odd when walking. It is OK when I first get up, but after 5 mins the feeling of tightness returns - feels like my kneecap might pop off! Any advice appreciated.