'Love Is Blind's Brittany Wisniewski Blindsides Boyfriend With Mod Sun Hookup

"Love Is Blind" star Brittany Wisniewski shocked her boyfriend back home in Maryland when she hooked up with Mod Sun in Los Angeles ... 'cause we've learned the guy thought they were in a committed relationship.

Sources familiar with the situation tell TMZ … Brittany has a whole BF on the East Coast, a hunk named Rocky Hawkins, and they've been in each other's lives for 4 years romantically.

We're told Brittany and Rocky took some time apart for 6 months in the past but had reconnected before she left Maryland to go film "Love Is Blind" for two weeks ... with Rocky driving her to the airport to send her off.

Our sources say when the reality dating show finished filming, Rocky picked Brittany up at the airport and they got back together romantically ... with their reconnection well known to family and friends in Maryland.

We're told Brittany recently told Rocky she wanted to gain more social media followers in order to make money off brand deals, so she signed up for 'LIB' ... and wasn't on the show to actually find love because she was in a romantic relationship with Rocky.

We're told Rocky drove Brittany to the airport this past Monday for her flight to Los Angeles ... and our sources say she told him she was coming to L.A. because Snapchat invited her out to tour their headquarters as part of her goal to build her social media following.

Our sources say Brittany told Rocky she was going to be hanging out with another 'LIB' alum ... and he was shocked to find out she hooked up with Mod Sun after taking in his performance Monday night at The Wiltern.

TMZ broke the story ... video shows Mod Sun and Brittany kissing and canoodling in a West Hollywood bar.

We're told Rocky was sitting in his office this week when his phone started blowing up ... people were sending him news article and photos of Brittany and Mod Sun together.

As we reported ... Brittany and Mod Sun also went out to dinner Tuesday night, holding hands on their way into a steakhouse.

Our sources say Rocky reached out to Brittany when he saw her with Mod Sun ... but she has yet to respond.

Rocky, we're told, is in absolute shock ... and our sources say the feeling is shared among their friend group.

We reached out to Rocky, who tells TMZ ... "We were in a relationship prior and right after the show aired. We have knowingly been in a committed relationship. I have been completely blindsided by all this leading up to when I dropped her off at the airport this past Monday for her trip to L.A. There's nothing else to it."