What Causes Bleeding After Intercourse?

I have been going out with my girldfriend for 3 months. She was not a virgin when I started sleping with her 3 months ago, but bled a little during sex. It got better and easier and we always use condoms up until last week.

However, last night she bled during sex and she has continued bleeding quite a bit during the last 24 hours. She has been in some discomfort and is naturally worried. She has been advised to go for a smear test as soon as possible. Could it be an STI otherwise?