keep asking the important questions -

how well do I feel?
how do I sleep?
how mentally alert am I?

please note that the use of muscles is bound to set off a reaction in your body to retain water near the muscles. if eating low carb, you cannot really gain fat, though you may easily gain weight. water intake and output really does not go hand in hand. you may be retaining water because your body needed it to thrive. now you are thriving, and more of life is occurring. it is possible that just this is responsible for building lean mass. beyond this, there is a likely gain in muscle if you do any weight training (low rep hard work). keeping these things in mind will get you through.