Having just returned from a ski trip to Big Bear just outside of LA, Selena Gomez and friends went to pick up some take-out for dinner after the group returned home.

Selena's actually been back in LA for at least a week following release from a treatment center where she was seeking help for mental health issues. She's been lying low and staying off social media ... until Tuesday, when her friends posted pix of the friends' trip to the mountains.

Meanwhile, her ex Justin Bieber's been exhibiting some strange behavior as of late ... we caught the emotional pop star shaking and bobbing his head back and forth repeatedly in his car yesterday as he was pulled over on the side of the road. It's certainly not the first time we've noticed Justin with this nervous twitch -- something that seems to go beyond a nervous twitch, actually. We're actually worried for him!

After stopping for coffee before an afternoon workout Wednesday, Bieber headed to Compton -- not sure why.

Then Wednesday night he hit up Hillsong's weekly evening service ... without wife Hailey Baldwin. We're pretty sure she's in town but busy working so Biebs hit up the religious revival solo. What if Selena had been there?!!!