I've been seeing this reaction image on /b/ and /jp/ but i can't find the source.
it's from one of rifyu's umineko 4komas
That's Ange Ushiromiya from Umineko No Naku Koro Ni.
Here's a link to the Danbooru pool with all of those. Fair warning, they are a lot more enjoyable if you have actually read the Umineko no Naku Koro ni visual novel (which is in English).
https://danbooru.donmai.us/pools/1313
Rifyu is pretty brilliant. Creator of such classic lines as "What a coincidence, it's masturbation time for me too!"