Usually female population is naturally relatively protected during reproductive age say 15 to 45 years but this is not 100% so there are many cases at younger age also commom predisposing factors could be Smoking,bad family history,Lipid disorders,some coagulation disorder and some times long term use of Oral contraceptive may be culprit. But at 19 years you should take a medical check up.If nothing significant then play some sport and pay attention to studies.If by chance smoking is there quit it.