What Causes Vaginal Bleeding After Having Sex?

I wanted to know what causes the bleeding. I only had sex with one person, my husband. We had sex for 4 times already. The first time we had sex, there was spotting and we thought that it is normal since I am virgin and so does he. The second time, there was no spotting, the third time, a spotting too. But after a week of no having sex, we had sex again and I had bleeding again. i wanted to know if it's normal since this is our first time. Thanks!