"Urm, who or what's behind you?" one asked.
"Can anyone else see the scary guy in the mask behind her?" another said.
"Why is there a dude with his mouth covered in the background?" someone else wrote.
Others called the sighting "scary" and "eerie".
The pictures – which show Lopez posing in her gym wearing a camouflage-patterned crop top and matching leggings from Niyama Sol – were posted on Sunday with the caption:
"If it doesn't challenge you, it doesn't change you," Lopez wrote in the caption.
The post has since been liked by close to three million people, with the star's followers initially losing it over how good she looked.
"GOD IS A WOMAN & HER NAME IS JLO!!!!!" one person wrote, while another added, "I need to come workout with you and get back into shape."
Earlier this year JLo stunned fans with her halftime performance at the Superbowl alongside Shakira.
The duo showing off some serious dance moves – and toned figures – as they sung a medley of their hit songs.
Lopez's most recent silver screen appearance saw her play a stripper in Hustlers, with the star taking pole dancing lessons on top of her regular gym workout.
JLo also changed her diet for the role, revealing she gave up carbs and sugar, however, admits the change was gruelling.
"(The challenge) was really hard. Not only do you get a headache, but you feel like you're in an alternate reality or universe. Like, you don't feel like yourself. You realise that you're addicted to sugar," JLo told Ellen DeGeneres.
Lopez is no stranger to sharing sweaty gym selfies of her six pack on Instagram and has credited daily workouts with keeping her happy – and ripped.
"Very rarely will I skip my workout," she told Us Weekly.
"Sometimes, I work too late the night before, and I'm like, 'Ugh, I can't do this.' But I tell myself, 'Just do it. It's only an hour.' It's just talking yourself off the ledge of being a lazy bum."
