The Little Mix singer, 25, posted a picture of herself with a Nickelodeon Kids' Choice Award and thanked fans for helping her band to win.
She wrote: “You only went and b***** did it! Thank you so much to our incredible fans for making this happen tonight we truly are the luckiest girls in the world we love you so flippin much.”
Clark commented on the picture writing, “My girl” followed by a heart emoji and clapping hands.
Nelson tagged him in her comment and replied with a three heart emojis.
It comes days after the Shout Out To My Ex singer posted a smiley face emoji with love hearts in its eyes on a picture Clark had uploaded to his Instagram account.
The celebrity pair first fuelled rumours of a relationship when they were pictured posing together at London’s O2 Arena after Little Mix won their first Brit Award last month.
Clark shared the picture on Instagram alongside the caption: “I don't want this girl to go back America tomorrow!”
Nelson was previously engaged to fiancé Jake Roche, but the pair split in November.
Speaking on radio show Z100 Nelson said: “When you break up with someone it’s awful. It’s like one of the worst pains. It’s horrendous.
“We all love to have some kind of sad song that we sit and cry to, but at the end of the day you need something that makes you feel empowered and think: ‘I don’t need you anymore. I’m going to go out with my girls and my boys and I’m going to get over you’.
“That’s exactly what Shout Out To My Ex does.”