Heavy bleeding but no miscarriage? Long.

I'm 13 weeks pregnant and today at work around 10:30 am I was talking to a coworker when all of a sudden it felt like I was peeing my pants. It just felt like a lot of wetness coming out at once. I went to the bathroom and my underwear and pants were saturated with bright red blood. I wiped and again a lot of blood. I left and went straight to the ER. By the time I got there the front seat of my car looked like I was in a blood bath. They did the routine stuff, asked questions, took blood, put in an IV for fluids, a pelvic exam, and an ultrasound. I had no symptoms other than the normal mild cramping and some nausea throughout the whole pregnancy. I've never spotted nor bled any. I'm not a smoker, haven't had sex for probably 2 weeks or longer, and at my last check up a week ago everything was great. Needless to say I was pretty shocked to go from feeling perfect to holy crap am I having a miscarriage? During the pelvic exam they said everything looked fine once they got all the blood out & they said my cervix was CLOSED?! After that I went for an ultrasound where I saw a baby but was thinking, is it dead? Is there a heartbeat? And then all you could see was a baby swimming around in there, moving its arms, moving its mouth. You could hear the heartbeat and it was measuring right on track. My labs came back and my quants weren't decreasing from my last appointment. I know some spotting and mild bleeding is normal during pregnancy although I never had any with my first. Since I've been home there has been no blood whatsoever. I was never really given an answer other than sometimes things happen. So I guess my question is, has anyone ever experienced this? How do you all of a sudden start gushing blood out of no where with a closed cervix and still have a healthy active baby? I'm so happy that I didn't miscarry but I'm beyond confused at the same time? Is there anyway to prevent this from happening again?

Sorry just need peace of mind.