It's most likely shedding leaves in response to growing in low light conditions. It takes energy to sustain that many leaves, and it can't do that if it isn't receiving sufficient sunlight. I grow mine in direct sunlight almost all year long. At least until it comes back indoors for the winter. It does perfectly fine, just so long as I keep it well watered.

Since yours isn't used to that much sunlight, I would suggest increasing the light it does get to bright, indirect sunlight. This will give it more of the energy it needs.

Thanks so much for all the advice, the window faces North West, but I'm in South Africa so that means afternoon sun. The room is quite bright during the day, but I will open the curtain a bit to let in more light.