I been on metformin for about two years and still 19 stone and have not lost any weight which i really should do.
Good luck with your weight, the only problem ahead is the more you eat, the more your blood sugar will rise. Like it or not its hard to have meals without some carbs or hidden sugars.
The other way of thinking is, your burning more calories than your eating, hence the weight lose. Have a think on how you can cut down on your hectic life style which in turn "hopefully" will start to slow down your metabolism.
If you are thinking of weight training to gain weight, here is some tips.
Eat small meals with no more than 30 grams of protein in every three hours as your body can only digests food once eaten in approx 3 hrs. Any excess protein will add strain on your organs and either be stored as harmfull fat or removed from the body( so wasted) eat a balance diet. An example is if you take a strong vitamin pill, when you pee next your urine will smell and is often a bright yellow colour which is your body rejecting what it cant use.
With weights keep the weight low reps to 8-10 / 4 sets each body part with free weights not machines. stick to basic exercises and train each body part twice a week. This is a very basic way to train and a good starting point, the more you train or advanced you get, the more you need to alter and adjust your routine to keep the progress coming.
Very best of luck.
Lee