Ultrasound

We went in to check on baby as I was having some minor cramping, no spotting. They told us that baby is healthy and we saw the heartbeat! The days and weeks match up to be perfect with what my dates are but they said there was some bleeding below the placenta, what normally causes this? Is this something to be really worried about? I have bad anxiety so it's up in the air at the moment because I'm so frightened by what the nurse told me. Is Baby going to be okay? I want to make sure that everything is good and baby is ok. But bleeding under the placenta seems worriesum. It also was mentioned that heartbeat was 140 for 7 weeks 3 days. Is that good??