This is one of Gloria Trevi's heaviest stories, and it's told like a diary. A girl is alone with a boy, she's scared, "Apenas y dije no" (I barely said no), and afterwards his eyes and his goodbye tell her the truth: "él no sería para mí" (he wasn't going to be mine).
Then the plot turns. "Me cansé de esperar lo que llega cada mes" (I got tired of waiting for what comes every month) is a quiet way of saying her period never came, and "algo crece adentro de mí" (something is growing inside me) says the rest. The chorus is the title: "Me siento tan sola" (I feel so alone). It gets worse at home: "la casa tembló cuando papá se enteró" (the house shook when Dad found out), and he orders "Lo debes perder" (You must lose it) so that "la gran sociedad" (high society, or simply what people will say) never finds out. She says the beatings hurt less than the loneliness.
The ending is her own decision: "Me dolió decidir que hoy me iré de aquí" (It hurt to decide I'm leaving here today), to a place "donde pueda nacer la inocencia" (where innocence can be born), meaning her baby. Notice the phrase "qué más da", which means "what does it matter". It's a brave song about a single mother, and in this live version she even dedicates it to all the single moms in the crowd.