It’s usually best to bring it up when it’s relevant to the current scene, unless you’re trying to make a point that the memories are invasive. You can create a small scene for this that also reveals something else about your character.
You can also reveal the information in steps so that it feels more natural. Think about when you remember an event that happened to you: you don’t have an internal two-minute monologue about the time you took a vacation. You remember bits and pieces that create a feeling. To replicate that feeling, try something like this:
Amy tapped her fingers nervously on her desk, gazing out the window at the concrete wall across the alley. It was almost 4:30 and she hadn’t so much as flipped a page in the file open in front of her in ten minutes. She cast a worried glance over her shoulder at the department manager and quickly straightened the papers before cramming the file into her bottom drawer.
Four of the other girls were going to the bar after work and Amy only had a small window of time she could escape in before they showed up in her cubicle to drag her away. They couldn’t see her - she was running out of excuses and although she wasn’t sure, her dog may have died twice in the last six months.
In a later scene:
The waiter handed her a short, wide glass and glared at her. She could hardly blame him - three drinks and not one tip. But what was she supposed to do? She was already spending money she didn’t have on drinks she wasn’t drinking. Amy tried to pretend that it wasn’t there, that her heart wasn’t pounding with stress, but the glass was slippery and cold and she couldn’t help but let the smooth scent of gin waft up her nose. The walk back to the crowded table was too long.
The first scene reveals some information about the character:
- Amy has an office job, and probably not a high-flying one judging by her view and her worry about her manager
- Her coworkers like her enough to invite her out repeatedly even though she keeps turning them down
- She’s a sloppy liar, or lies so much that she can’t keep track of it
- She’s been working there for at least six months
- Something about going to the bar or out with her coworkers makes her nervous
The second scene expands upon that by revealing that
- She didn’t escape/couldn’t lie her way out of going
- She doesn’t have money to blow on drinks
- She’s not drinking them but finds them enticing, which is setting you up to reveal in more detail at some point that she’s a recovering alcoholic (and for whatever reason, secretive about it)
You can reveal just about anything about a character this way without having the scene be clunky and out of place. When I’m doing exposition, I always like to make sure a scene is revealing at least two things about a character’s personality, motivations, past, etc. Scenes that exist only for one purpose often feel flat to me. It’s interesting to read because it lets the reader deduce a bit on their own, which is more like what we would actually experience if we were discovering something about a real person. Your approach will need to vary a bit depending how explicit or surprising you want to be, but hopefully this gives you a starting point!