Miscarriage at 7 weeks (possible TMI)

Stephanie
Last night it was confirmed that I had MC'ed.  I was "officially" 7 weeks on 5/1, due 12/18/15.  I had my first prenatal appointment on Tuesday, 4/28, and everything was fine.  I did have some slight spotting, but very light and it was brown, so no one was too alarmed.  The doctor still prescribed for me to have a transvaginal ultrasound.  My husband and I went on Wednesday, 4/30.  We saw the heartbeat.  We were told the crown rump length was where it should be at the time - 6w and some odd days.  There was a sac of fluid noticed, containing about 1 cc of old blood.  We were also told that the gestational sac was measuring out to just over 4 weeks.  However, the doctor told us with the crown rump length and the heartbeat, the chance of MC diminished, although he still couldn't guarantee anything.  I had a follow up ultrasound scheduled for next week.  
I didn't have any additional spotting, no cramping, no pain at all.  Then, on Saturday, 5/2, after going to the bathroom, I noticed a large amount of blood in the toilet, almost as if I had my period.  I called my doctor and she told me if I soak a pad an hour to go to the ER.  Well, rather than waiting because I had a bad feeling, I headed to the ER anyway, even though there was hardly any blood on the pad, only when I wiped.  
They did a pelvic exam and ultrasound.  During the pelvic exam, the doctor noted there were some blood clots, but my cervix was closed.  However, when they did the ultrasound, it was confirmed I had MC'ed.  There was no sign of a gestational sac.  
I'm still pretty numb and in shock.  I can't believe how quickly this happened.  My OB/GYN was equally as surprised, particularly because of us seeing the heartbeat on Wednesday, then nothing happened until last night.  Plus the fact I had no symptoms that would indicate I had MC'ed until the blood.  
At this point, we are just going to measure my HCG levels until they get back to 0.  I was already down to 600, so it shouldn't take too long.  I just can't fathom how quickly this happened.  
Has anyone else had an experience similar to this?  Did you do a follow-up exam to ensure everything passed?  My doctor didn't seem worried that I hadn't or wouldn't pass everything since there was no sign of the gestational sac on the ultrasound.  This was our first pregnancy.   I am a pretty healthy person as far as nutrition and exercise.  I mean, I train for half marathons.  My blood work and all of my exams through the years have been stellar - off the charts good.  I know this isn't my fault, but the fact that this happened so quickly without any warning signs is scary and disheartening.  I was told there isn't any reason why I would have issue in the future with getting pregnant and carrying a baby to term.  At this point, I'm just feeling alone and am looking for someone with a similar story or any sort of insight.  
Sorry for how long this story ended up being.    I just needed to get it all out.  I also apologize for not using the acronyms - this is my first post and I'm not 100% familiar with all of them.