hi and welcome to the MS forum. Before we can begin to guess at an answer for you or suggestions as to what to ask your doctor, it would be helpful to know a bit more about you. Are you diagnosed with MS? If so, what are your other symptoms you have?
I'll watch for more of an explanation.
weakness in the legs and passing out are two separate issues.
are your legs weak like they are giving out or do you feel dizzy like fainting on exert?