The reason for the first question is that, believe it or not, it's pretty common for women under the age of 20 to experience irregular periods. Even though you might be all finished with puberty, there are still drastic changes in your hormone levels at this time due to a variety of factors. In fact, I've even heard some stories where the women really never got to be 'regular' until after their first child was born.
On the flip side though, you could have a condition called menorrhagia which means excessive uterine bleeding. It can be caused by a variety of factors; one of the most serious being uterine tumors (not as uncommon as you think...I've known two women under the age of 25 to have been diagnosed with them). The only way to be certain is to visit a gynocologist and tell them what's going on. In the meantime, you might want to check out this link here:
Oh, and on the sex thing...women will sometimes bleed a little after sex if their hymen hasn't fully been broken or if she wasn't lubricated well. Again, the only way to be certain is through a gynological exam. [ MissNiceness's advice column | Ask MissNiceness A Question ]
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