Just did a vaginal ultrasound. found twins but no heartbeats. no heavy bleeding or anything. could it just be too early to detect them?

Unlikely: If the embryos are at least 6 wks gestational age a decent us should be able to detect cardiac activity. Hopefully it is earlier than this and a repeat will reveal good news. If not, follow up with your OB and remember that sometimes nature selects abnormal pregnancies or chromosome problems and simply doesn't allow a pregnancy to progress. Ie - there is no way for you to prevent or cause this.