loosing weight may help with falling pg - loosing as little as 10% of your bodyweight has been shown to have benefits. But it does sound as if something else may be going on in your body, so hopefully the doc will be able to find out what may be causing the irregular periods and have some options for you. But it is possible to fall pg while overweight - I myself was 30kg overweight when I fell pg with my first.

Good luck TTC