Trouble in paradise? Justin Bieber and Hailey Baldwin appeared to be getting into just before a gym session on Monday, and Justin even took some of his stress out on the paps, yelling at them for privacy as he had a SERIOUS "discussion" with his wife.

Is it wedding planning that's elevating the tension? The pair are said to have decided on September as their wedding month, according to TMZ sources -- but just not the exact day. But seriously, if the wedding's potentially less than 5 weeks away, how could they not have nailed down a date?!

Whatever it is, the two were getting heated -- and it's not the first time we've seen them this way recently.

Earlier in the afternoon they visited the dermatologist together -- the go literally every few weeks ... for what? Botox? Laser treatment? Hailey took the lead and Justin lagged behind, both with coffees in hand. Hailey was still in her yoga gear from a morning workout with pal Kendall Jenner while Justin wore a preppy-cool layered polo shirt, New Balance sneakers and a cap.

Question is ... will these two walk down the aisle next month?!