Kashimashi - Girl Meets Girl

Synopsis

From Seven Seas: Being a girl is harder than it looks. For Hazumu, this couldn't be more true, because just the other day, she was a he. Shunned by the girl of his dreams, Hazumu loses himself in the mountains and is promptly squashed by an oncoming spaceship. The alien inside, feeling guilty, rebuilds Hazumu's body but as the wrong gender! Now Hazumu must learn how to be the girl his parents always wanted while dealing with the trials and tribulations of being caught in a love triangle between two girls. His childhood friend, Tomari, and Yasuna, the girl who rejected him but is now strangely attracted to him/her!

Synopsis & details from MangaUpdates