Bleeding after sex

To make a long story short my boyfriend and I had sex for the first time and I was in pain. When he pulled out he noticed I was bleeding. I had blood clots the size of my fist leaking out for almost 20 minutes.Also I felt light headed and dizzy. After that I started my period. I talked to my doctor and they ran some test and told me everything’s good down there, but I’m scared it will happen again . Does anyone know what this could be?! Or if sex will be more pleasurable when I decide to try it again?