How much reputation did you lose today?
JavaScript ES6, 132 bytes

n=&gt;fetch(`/ajax/users/${n}/rep/day/1497916800`).then(v=&gt;v.text()).then(v=&gt;alert((v.match(/rep-down"&gt;-(\d+)[^u]+user w/)||[0,0])[1]))


Tested on Safari 10. This uses the new fetch API and alerts the output. This is a function so pass the user id as the argument. This will output 0 for no rep changes.

Must be run on PPCG due to CORS (Cross-origin resource sharing), browsers do not allow JavaScript to perform a web request outside of the current domain.