It sounds as though you could possibly be showing some early signs of MG. I would recommend you begin collecting some photos of yourself when you are both strong and when feeling the weakness. To be the most accurate, choose a place where the lighting and background can be consistent. You may also want to keep a symptom diary. Many women notice a worsening of symptoms associated with our menstrual cycles.

Once/if you collect enough information to make you feel that yes, this is a problem, you can present the information to your doc again and ask for a referral to a neuro or ophthalmologist. Of course if anything gets much worse (as it may with warmer temperatures) you may need to act sooner.

Good luck!