Isotretinoin has many bad
side effects, but muscle weakness is not one. It has so many
side effects, I would be reluctant to take this medication. You can read the entire list of negative side-effects online -- just search for isotretinoin.
However, since you're finished with your fifteen day course without incident, your risk in using this medication is greatly minimized.
If you had a serious infection, this would of course weaken you. With constant pain, energy is always affected.