Hi. I am 19 years of age. Back in February, I had three menstrual cycles in a 6 week time span; approx. 2 weeks apart. Then, didn't have another after those 3 until May 2nd. It's now September 1st and I haven't had a period since then. I have taken one pregnancy test, and it was negative. I am sexually active, not on birth control but have always used condoms. I haven't had any symptoms AT ALL of pregnancy either. Also, I woke up on August 30th and August 31st, and also today, where I had severe abdomenal and back pain, and my belly area seemed firm. Really firm, and any pressure hurts to touch. Don't have insurance at the moment... Please help :(

I'd try to find a free or low cost clinic. Are you in college?

Abdominal Rigidity: Causes, Symptoms & Diagnosis

As you can see, there are MANY possible causes, and many are serious, so go to an ER. I rarely suggest that.
Having said that, a common condition in women with your symptoms (if not pregnant, not ectopic pregnancy, and not STDs) might be PCOS. You can research it online.

If it's crowded because of the holiday (US) and you aren't faint or feverish and can wait until tomorrow, do so, but be prepared to go at any time if anything worsens markedly.

You really should visit a doctor. The Internet is not the best place for diagnosing your problem. I hope everything is fine.