Is it possible to have acute gastritis without pain? After drinking, I threw up blood but did not have any pain, just nausea and loss of appetite.

Gastritis. You may have gastric distal esophageal irritation erosion / reflux without significant pain and the alcohol caused the eroded area to bleed. Would follow up witht GI may benefit from endoscopy , checking for H pylori and treatment with ranitidine/ prevecid which in the US is over the counter.