My period was 2 weeks late but when it finally came it was really heavy and now a week later i have horrible cramps and dizziness. why? =\

Could be hormonal: I am assuming you are not on any hormone medication (ie birth control pills). Since you are 20 you could still be prone to what we call anovulatory cycles where there will be some months where your ovulation is off or does not occur. These cycles can skip or sometimes be unusally light or heavy or crampy. Should be safe to follow for several months and see if regulates out.