Knee Cap Displacement. Of my problems?

In last October, i was sleeping on floor. When i tried to get up, somehow my knee cap got displaced suddenly. I somehow pushed it into its place. Severe pain was there at that time. Later i consulted one ortho, he too confirmed after looking at xrays and all. He advised me wear a bandage for three weeks and gave some medication. After that time period my pain ceased and during next checkup the doctor said everything is right now. But still i feel uncomfortable when ever i take stairs or whenever i get down to pickup something on the floor. Light pain is also there. And also i can't walk bare foot for long. I need to use cushiony shoes/sandals to feel comfortable. I have a flat foot. Is there anything i can do to get rid of this, and make my knee as good as it was before this incident? Without a proper shoe i can't walk for long distances without feeling leg pain afterwards. Is my flat foot is the reason for my knee issue? How to get rid of my problems?