If it happens here and there, its usually nothing to be concerned about. You could simply need more lubrication or could just be caused by too much friction.
It could also be cysts or polyps.
For me, I bled A LOT during sex and it was due to cervical polyps. I first had a D&C which took care of the issue for about 6 months. They grew back and then I had to get endo ablation which made it stop, along with my period and ability to carry a baby.
I would recommend visiting your dr and getting checked out. There are many reasons for bleeding during sex, some nothing to worry about, others more serious. It could be something as simple as your boyfriend is large or you were a little over-enthusiastic and he caused a tear which will heal itself, it could be something internal like a fibroid or endometriosis, it could be an infection, or, although unlikely, it is also a symptom of abnormal cells in the cervix.
I'm not sure where the other post is that was mentioned but you didn't mention how old you are, whether you're regularly sexually active, whether your periods are regular etc. Although it is unlikely you would become pregnant 5 days before your expected period, it is possible. If your periods aren't always on time, your ovulation day could be later.
You said in your other post that this was five days before you expected your period? If so, you won't be at risk of pregnancy, since you would not be ovulating 5 days before your next period was due, more like 14 days before.