Early sexual intercourse can result because of bullying, sexual harassment (that is unwanted from both sexes), and drugs. Social lives put peer pressure on someone and makes them want to do things the cool kids do or there forced to do.
Teens who engage in sexual intercourse early might end up being bullied and labelled in a negative way as unfortunately we live in a society that does that. Friends and family might turn their backs on them and make them feel socially isolated. They could end up pregnant and still have to attend school while pregnant and deal with extra abuse from other kids and possibly teachers.
The CDC suggests retesting because of the "window period." This is the length of time between exposure to HIV and when HIV shows up on a test. This can take up to three months. So if your test result is negative, protect yourself and get retested after three months.