Im told i have mild lateral rectus weakness in both eyes. what is it? i have a possible diagnosis of mg responsive to mestinon & a confirmed diagnosis of behets which one of these is more likely to cause this? is it something i need to worry about?

The lateral rectus is an extraocular muscle allowing the eye to turn to the side. Weakness on lateral gaze results in double vision. This can occur with either Behcet's or Myasthenia Gravis, and a positive response to Mestinon would suggest the latter diagnosis. Yes, you should be concerned and follow up closely with your treating neurologist.