Looks like there's trouble in the ballroom for Vanessa Lachey and her Dancing With the Stars partner, Maksim Chmerkovskiy.
Lachey danced with pro Alan Bersten on Monday night's DWTS, after he surprisingly stepped in for Chmerkovskiy. Co-host Erin Andrews said Chmerkovskiy's absence was due to a "personal issue."
A source tells ET that he actually refused to rehearse with Lachey this past week due to the two not getting along.
According to our source, both Lachey and Chmerkovskiy are "stubborn, strong-willed people," and it was obvious from the beginning that they were going to "butt heads." The source also claims the pair has been faking their seemingly close relationship.
"They're both super competitive and know how to 'turn it on' as performers," the source adds.
Alan Bersten Steps in for Maks Chmerkovskiy on 'DWTS' as the Pro Deals With a 'Personal Issue'
The source says their issues "erupted" this past week, resulting in Chmerkovskiy's refusal to rehearse with Lachey in preparation for Monday's episode.
"It was nothing specific, just little disagreements that escalated," the source says.
However, the source notes that Chmerkovskiy will be back next week.
"He'll have to put his ego aside because there are contracts involved," the source says. "He can't just walk off and leave her hanging. One week, sure, he can just say 'personal problems,' but he'll be back next week."
A DWTS spokesperson also tells ET, "Maks and Vanessa are returning to rehearsals and will be dancing together on next week's show."
Meanwhile, on social media, there appears to be nothing but love between the two partners. On Monday night, Lachey Instagrammed that she "can't wait" to dance with Chmerkovskiy again.
"Thank YOU to @alanbersten and my new girl posse, the @dancingabc girls troop, @brittbenae @hayley.erbert @dance10jenna!!!" Lachey captioned a pic of her hugging Bersten. "This was the ULTIMATE guilty pleasure! #GirlsNight Don't forget to vote!"
"I can't WAIT to get started tomorrow with @maksimc on week 4!" she added. "We're cookin up something special for you!"
And earlier this month, Chmerkovskiy praised Lachey as "one tough chick" on Instagram, sharing video of him bandaging her bloody toe as she cried.
Earlier this month, ET spoke to both Lachey and her husband, Nick Lachey, who's also competing on this season's DWTS. Interestingly enough, the former 98 Degrees frontman is partnered with Chmerkovskiy's wife, Peta Murgatroyd.