It looks to me like over-watering. The white powder looking stuff on the top of the soil is an indicator that the soil has not gotten a chance to dry out well enough. Give it 2 - 3 days, then check moisture by putting your finger in the soil to the second knuckle. If you feel no moisture, then water, but only give it a little bit in the center of the pot (making sure it doen't just flow down the side of the pot). At that size, I give my plants ~150ml of water with nutes. I think it is best not to saturate the entire pot, because doing that cuts off oxygen to the roots.
If you decide to transplant (it looks to me like you have plenty of room in that pot) use very loose soil. Vermiculite is great. Composted pine bark is awesome, but sometimes hard to find. Also consider using plastic pots (I thought yours looked like clay, but couldn't really tell). With plastic pots it is easier to judge when to water by its weight.
Lastly I would advise you to not go into flower until the plant is back into good health, so as to not stunt any bud growth.