Female dog - hind legs

I have 3 years old American bully female. She was very normal and active and also had a beautiful litter a year ago.
During past couple of months she is showing some strange difficulty such as - following to a long walk she’s finding hard with her hind legs for one or two days that she’s extremely exhausted and she hardly uses her hind legs to walking. She sometimes walk only by using her front legs by dragging the hind legs on the floor in sitting mode.
Dear experts please explain this situation to me. When I took her to doctors they said all her health was in an order and there’s nothing bad to mention.
I am restless with this situation.
Please help me.