probably too much fertilizer.

which direction is the window facing?

I am a bit confused about your posting as it is not clear from the picture what the problem is. Watering once a week, in the Summer sounds low, especially in a gritty mix (but realize this depends on sun, temperature, humidity, etc). One more comment. Are you sure that is a Blood Orange?! I have two if them and neither has thorns like that.

What problem is the tree suffering from?

Well she's not outside. She's in a west facing window that gets evening sun. She's a Tarocco Blood Orange tree. I purchased her from Four Winds. Her issue is some color change in the leaves and they are just shriveling up and falling off. They also come in distorted and don't have a nice glossy look. I guess it could be too much fertilizer but I figured it was the proper feeding schedule.