Pain for several days on left towards back of where leg meets lower hip area and pain getting worse and feels like leg is about to give out. Causes?

Hard to say. Many things can cause pain and weakness. Much more history is needed as well as an exam to be able to answer that well. It could be muscular such as a strain, or bone from trauma, or even infection. With several days of pain and a sense of giving out, it is time to be seen!