I'm looking for a manga about a girl who can smell when people are sexually aroused
I'm pretty sure it's Para Paru :)

It is a shoujo manga about a girl who wakes up one day finding herself being able to smell certain scents from the people around her. She also hears a strange voice inside her head. Later with the help of a friend, she finds out that an alien got inside and is now living in her head, and it is the one responsible for giving her the enhanced ability of smell. The certain scents are explained and analysed to be associated to sexual arousal and physical attraction, as well as dishonesty and nervousness, etc.

There are other people in the same school who receive similar abilities in taste, touch and hearing. The manga is ongoing and touches on sensitive and mature matters such as rape and pregnancy.