I wish I knew why 😭

I had a miscarriage in May at 19 weeks. I had a healthy pregnancy and the doctors never really gave me closure to why it happened except it might have been because I was bleeding. The bleeding first started at 11 weeks while we were having sex. I was on pelvic rest till it stopped then the doctor said it was ok. We had sex and I bled like crazy with clots and everything at 14 weeks. To my surprise my little girl was fine but I never stopped bleeding. I was put on pelvic rest until it finally happened and I started having contractions. My baby had stopped growing at 16w4d. I never stopped thinking about it because I don’t know what happened and I never got any answers ever. If doctors say that sex is fine during pregnancy than why do they put you on pelvic rest (no sex) right away when something happens?