Late Period

So I had unprotected sex earlier this month with my boyfriend and I thought everything was okay until my period was late.  My cycle is never to the day though and can often be late or early so I didn't worry much. But after a couple days I started freaking out because I'm only 16 and I did nooot want to deal with an unwanted pregnancy. This morning I went to the toilet and noticed blood in my pants and I can safely saw this is the only time I have ever been thankful for getting a period. Have any of you ever had similar experiences?