Hymen didn't break

Okay so my boyfriend took my virginity but I didnt bleed, I know it's not because of his size because he's big enough to break my hymen for sure. We've had sex quite a few times and he was only gentle the first time, it hurt for the first 2 seconds everytime though and I thought something was wrong. Well yesterday I don't know what happened but I know it was my hymen breaking, I felt a pop and there was blood everywhere. Why didn't my hymen break the first time we had sex or any times after that but it did yesterday?????