When I Have To Sit On The Toilet For More

When I have to sit on the toilet for more than a second my feet go numb. I've learned I have to get some feeling back in them before standing because sometimes my ankle gives away and I'm worried about breaking my ankle if that happens again. My bigger issue is my knees. Sometimes after sitting on any hard chair I have to be very careful because my knee(s) will pop out backwards. I think it's called hypertension. So when I go to make a turn I have to be very deliberate for the first few steps of the turn. I have also had the disappearing leg, when I am walking the leg getting ready to take the weight and balance just simply goes away. No pain, just vanishing.