The phrase, "Soy in Madrid" does not make sense.
The phrase could either be "Estoy en Madrid" or "Soy de Madrid". Moreover, the lack of punctuation makes it confusing. Do you mean, "Estoy en Madrid, mi amor. Te amo, mi amor."; or "Soy de Madrid, mi amor. Te amo, mi amor."; or as is? If the first option as in "Estoy en Madrid", then the sentence means, "I am in Madrid, my love. I love you, my love." The latter means "I am from Madrid, my love. I love you, my love." Depending on you are talking to you might want to say, "Te quiero" instead.

"Mi corazón" is almost appropriate for, "my sweetheart".