Grandmother got a Hip Replacement the other day. Now is suffering from dementia. Any ideas?

My grandmother lost her husband a few months ago and ever since has been suffering from dementia. But a few days ago she got a hip replacement and in the past few days has been out of it. She talks like she is drugged, and just seems like shes in a whole nother world. I have looked around and found that this is common in people who have hip replacements. Does anyone have any idea why this may be happening or what can be done to prevent it?