Spotting for 2 weeks than heavy the next week with blood clots (not normal for my period) and whenever I get horny I bleed. Why?

Hormonal imbalance. Menstrual irregularities are not uncommon and anovulatory cycle is a common cause. If you miss more than two periods it would be time to see your doctor. If you do not wish to be pregnant, use contraception all the time. You may consider implanted contraceptive or an IUD.
Consult this site for more information on this topic.
http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/Pages/causes.aspx
Practice safe sex, if you have sex.
Get HPV vaccine.
DYSMENNORHEA. There are many possible explanations for your symptoms, ranging form anovulatory cycles, to hormonal shifts to polyps. Also you need to rule out early pregnancy. Should this continue, go to an obgyn for evaluation.