Sounds like you have a hamstring tear. Use the search function or google "starr rehab protocol". Use deadlift as the rehab exercise. Start the protocol 48-72 post incident. Muscle injuries don't always have a predictable cause and sometimes just happen for no good reason. This means they are very hard to prevent. The good news is, when there isn't a rupture, they heal pretty fast and the starr protocol works very well.