Margaret, try not to worry about all of this, including the "giving way." You are still very early in recovery. Some folks have a slower time of it than others. You have said your quads were lagging, so that could certainly contribute to what happened today. Just keep doing gentle exercises....don't PUSH too hard. Remember, you can't speed up recovery with exercise....quite the opposite, actually.
Also, if you aren't using a cane or crutch, I would do that. It offers a bit of extra support for you right now. Once you are stronger and walking with a good gait, you can and will leave it behind.