Kyou Wa Kaisha Yasumimasu.

From MangaHelpers:
Aoishi Hanae is a 33-year-old woman who has a job, but no boyfriend. In fact, she's also still a virgin. It's not that she never had a chance to lose her virginity, but she let the chance pass by. At a drinking party with her colleagues, Hanae ends up confiding in Tanokura Yuto, a handsome young 21-year-old part timer at her company who also attends college, and who has recently broken up with his girlfriend.
The next morning, she wakes up in a hotel next to him, and learns that they apparently agreed to start dating and then had sex that night. She has only hazy memories, and has no idea how to deal with the situation. Yuto is shocked that she doesn't remember, but hopes that they can still date. How will Hanae, who is older but much less experienced in romance, handle her surprising new relationship with the younger and more experienced Yuto? [tethydust]