yeast infections and bacterial infections can both cause bleeding after intercourse. I agree with msniki. go to the doctor so you don't continue to worry over something that probably isn't even a big deal. good luck!
You need to see your GYN due to abnormal bleeding. You should not be bleeding like that after having sex. It could be a number of things as to why your bleeding, but first thing is to see your doctor. There is nothing to feel ashamed of, many ladies have these issues with abnormal bleeding but you need to get it taken care of.