When you first start menstruating, the schedule can be all over the place. A typical "normal" cycle is between 21 and 35 days so 14 to 20 days is short...except you also need to count the days you are actually menstruating.
The concern is that heavy bleeding every 14 to 20 days could very likely lead to anemia eventually so yes, this should be checked out medically.
Is there a Planned Parenthood Organization near where you live? If so, they offer lower cost reproductive services such as gynecological examinations that may help diagnose why this abnormal bleeding.
The risk from all this bleeding (besides the fact that it MAY...hopefully not...be caused by something serious) is that if it continues you will become anemic. The good news is that in the vast majority of these cases the cause of this bleeding is usually not a serious medical reason.