OUCH!! Glad you were able to be seen so quickly and that the shot worked fast too. I lived in that neighborhood and fortunately never needed immediate medical attention, but it's nice to know it was there if we needed it.
In the book Four Seasons in Rome by Anthony Doerr (read my short review here
Recommended: Four Seasons in Rome), he too lived in Trastevere and tells of the time his wife passed out and needed emergency care. They are an American family with limited Italian skills, but were treated quickly and professionally. Turns out the wife, a mother of infant twins was suffering from exhaustion and dehydration - no time to take care of herself. Their medical bills totaled zero as well, and if I remember correctly that included a day or two in hospital.