What could have caused me to lose my baby at 5 months?

I just lost my baby at 20 weeks and 4 days on Monday. I started bleeding (not very heavily, just a little) and it scared me so I went to the hospital. They did an ultrasound and checked for heartbeat. There was no movement nor heartbeat. They actually performed a D&C. The baby was so small (smaller than should have been) but at previous appointments everything looked fine and appeared as though he was forming properly, etc. They could not give me a reason for my loss. I am just wondering what the cause was. Could it have been chromosomal abnormalities? Could it be due to the fact that in the beginning (1st 2 months) of my pregnancy I consumed alcohol? I did not find out I was pregnant until 11 weeks and had regularly consumed alcoholic beverages up until that point. I was also extremely stressed out. Had recently broken up with my boyfriend, my car had broken down and a long series of stressful events. I was very depressed. Was not eating well during the 1st two months (again I remind you, I was unaware of my pregnancy). Also, could the father have anything to do with it? He is a very heavy drinker, smokes and does not take care of himself at all. Just want an answer to why I lost my baby boy.