I'm a 18 year old girl. I had unprotected sex on June 23 and I had a period on the 25. I hadn't had a periodd since then but on August 18 I thought my period came on because I had a little bleeding but it lasted for like 5min and it was no more bleeding after that. I took a home pregnancy test and it said negative. Am I pregnant? What does all this mean?

You could just be spotting, you should go take another pragnancy test of a different brand, that one could tell otherwise!

First, any sex, protected or not, can result in pregnacny. I would be able to give you a better answer if you said whether the period you experienced on the 25th was when you were supposed to have a period or if it was just bleeding. I mainly just need to know when your period was due.

My period never comes on a certain date but it usually comes on toward the end of the month.

No, very few people have a period on a certain date, but know when to expect it... typically 28 days after the first day of your last period... don't you chart your cycle... if you don't, you should, so you know better how your body functions.

Or are you saying that you are irregular, meaning that some months you go 28 days and some month 33 and some months 20... etc..

The end of the month can mean anywhere from the 20-31... I would say that your chances for pregnancy are fairly low, as you most likely missed your most fertile time, but really it only takes one good swimmer.