Mainly a girl bled because her hymen (contrary to popular belief hymens only partly cover a woman's vagina) hadn't been stretched enough before (stretching can be done by fingering) so it tore. This is common so don't worry. Basically the hymen is much less flexible if it hadn't been stretched before hand. This is also why some women bleed if they hadn't had sex for several years as the hymen becomes less flexible again.
Did it hurt her when you guys were having sex? If it does or you notice bleeding while having intercourse then stop. Also, it could have resulted from not using enough lube. If it carries on bleeding for some time afterwards my advice would be to see a doctor. By the way, don't take this, either you or your partner, too badly because it is common (hence why many people believe every girl bleeds on her first time).
Also, did you really just use the phrase "break in a girl"???