Camila Cabello is spilling all about her brief reconciliation with Shawn Mendes -- including what caused their final split ... and the gist of it is, they just weren't right for each other.

The singer went on Alex Cooper's "Call Her Daddy" -- where she revealed that her reunion with Shawn ended as quickly as it began. Apparently ... she and SM realized that -- despite their hot make-out sesh at 2023's Coachella -- they weren't as good a fit as they thought.

Camila said she jumped into things again with him on an impulse -- but soon, she says they both realized it wasn't working and that they didn't need to force the relationship. CC recalled deciding just to be friends with Shawn ... saying she and SM ended on good terms.

The two pop stars previously dated between 2019 and 2021 -- before giving it another go ... which included a date night to Taylor Swift's "Eras" tour in May.

Doesn't sound like Camila regrets the rekindled romance ... calling it a "fun moment."

The fun didn't stop there for Camila, BTW ... who was spotted on vacation with Drake in Turks and Caicos this past December. However, Camila clarified that it wasn't all good times and games -- it was a work trip, too ... labeling it a "friends vacation."

Unfortunately, the singer stayed tight-lipped on whether she and Drizzy did more than make music. But, she did notably share ... "I will say I love him."

Shawn, for his part, has also moved on from Camila -- having been linked to British presenter Charlie Travers last fall. Remember, Shawn and Charlie were spotted enjoying a beach day in their underwear after linking up in West Hollywood in November.