I had Botox yesterday and now my upper eyelids are hollow and sunk in. What can cause this? (photos)

When the forehead is treated, the skin of the forehead can drop because the muscles are no longer able to hold up the skin. This can cause the heaviness and, in your case, the skin is collecting at the brow causing the eyes to look sunken. This will eventually wear off. Since you only had the treatment yesterday, it may get worse before it gets better. It is important to be very conservative when treating the forehead with neurotoxins. 

Honestly, it's too soon to see the effects of the Botox treatment and what you are noticing is probably not related to the treatment. The best thing to do is look at your before photos which hopefully were taken by your injector and will show that the hollowness was present prior to treatment. In certain cases, hyaluronic acid based fillers can help with periorbital volume loss.