Onzoushi to Keiyaku Omega: Ikezu ni Amaku Kamaretai

A world where alternative genders exist: Alpha, Beta, and Omega. Koyama Rio has lived as a Beta for 25 years. When she finds out she's actually an Omega and confesses to her boyfriend, she is brutally dumped. When she goes into heat, a state unique to Omegas, she is rescued from a dire situation by a complete stranger, the finest Alpha, Shinonome Sena.

But Rio's body responds to Sena, and her throbbing becomes even more intense...!?

(Source: Kodansha, translated)