What could cause menstrual irregularity?

So I had a period at the latter part of June....?
Which is normal for me, but then had one aqain July 12th through the 18th. And now it is August 14th and, did not have one. What could be causing irregular periods like this.

Additional Details

2 weeks ago
I forgot to add that I did have unprotected sex around the 8th of July. But I know there is no way I could be pregnant since I turned around and had a period days later

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Ok, that was my original question. It is now Sept. and I still have not had a period ( & no signs of a period ) , my stomach is hard, but I take the home pregnancy tests and they say negetive. Nor do I experience pregnancy sysmtoms. So what could be wrong? I am not on any birth control.
~Answers from nurse practitioners are welcomed~

Thanks, You all but I am not stressed out about anything, I am not on birth control and I normally have periods ever month, on time stressed or not...once again thanks but not correct answers. I will just go to the doctor as I should not expect correct answers from ~Yahoo Answers~

Just because you had your period doesn't mean you can't be pregnant. My friends sister went 8 months having a period and went to the hospital for bad stomach cramps, she thought it was appendicitis, turns out she was in labor! Go to the doctors and see if you are pregnant!

Have you been under a lot of stress lately?

Stress can be a major part of why your period hasn't come. I have recently experienced that, and my period came after 10 days. I know it's scary, because you feel unsafe and that something is wrong with you, but it will be okay. It was easier for me to not worry about my period because I have never had sex, but back to your problem...

Try going to a doctor and finding out what's wrong with you. Ask your mom to take you, and tell her what's going on. She can help with things like this...

Good luck.

I can't tell you why you are having these problems but when I had the same thing, my doctor put me on birth control for only three months and I have had a normal cycle ever since...so see your doctor or go to a free clinic...Good Luck!

It could be that you are just stressing. Stress can throw some women's periods off by weeks. Calm down!
Also, you don't say your age, but it could be you are just not very regular...and may not ever be.
Just because you don't have "pregnancy symptoms" does not mean that you are not pregnant! Some women have a period, or several, and are pregnant! A doctor's pregnancy test might be more acurate. Go to a clinic if you don't want to go to a doctor's office. They are usually free.
PLEASE, ALWAYS USE A CONDOM! It keeps you both safe from SO many things!
Number one rule, NO condom? NO SEX!

Menstrual irregularity is caused by many factors including, eating habits, stress and weight loss. It could also be caused by your hormones, which is very common in females. The best way to correct this mentstrual irregularity is by taking birth control pills, which will keep your hormones under control. This recommendation was offered by my gyno coz I have the same issue.

ok, well it could be a number of things, heavy work loads, lack of sleep, stress, a change in your diet, change in exersize routine, dramatic weight loss/gain or medication. I would go and see the doctor just to make sure that everything is ok. good Luck.xx