"All in My Head (Flex)" is a song recorded by American group Fifth Harmony featuring vocals by rapper Fetty Wap for their second studio album, 7/27, which was released on May 27, 2016. It was produced by Stargate and Brian "Peoples" Garcia with additional production by Sir Nolan. Musically, "All in My Head (Flex)" is a reggae-influenced pop song backed by synths, percussion, guitar and clips that contains an interpolation of the 1995 song "Flex" by Mad Cobra. The song received positive reviews from critics who praised its production and sensuality present in the lyrics. Following the successful chart performance of "Work from Home" the song was selected as second single debuting as an encore performance of the 2016 Billboard Music Awards. The single was serviced to rhythmic contemporary playlists in the US on May 31, 2016.

Commercially, the song reached number twenty-four on the United States' Billboard Hot 100 becoming Fifth Harmony's third highest position in the country. The song also reached top 30 in, Australia, New Zealand, United Kingdom, Canada and Ireland. To promote the song, the group performed on the season 22 finale of Dancing with the Stars. The single's accompanying music video was directed by Director X and was premiered at Vevo platform on June 23, 2016. The video was set on a beach and tropical environment, in the video the group dance on top of rocks and pose in different ways barefooted during their individual solos and wearing different swimsuits as they frolic on the sand and tides. "All in My Head" won the category Song of Summer at the 2016 MTV Video Music Awards.

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