I'm an active 18 year old girl. My blood pressure is 145/91. Is this strange or high enough to have to get it looked at?

Yes. You should see a doctor for that. Both the top (systolic) and bottom (diastolic) numbers are elevated. If you're that young with such an elevated pressure, you may need to alter your diet, stress level, or better manage other chronic illnesses. In addition, it may be due to endocrine or renal (kidney) issues that should be ruled out by your doctor.
Yes, too high. Try to relax when you have your blood pressure taken and no worry about the number. Make sure it is taken correctly. Talking, listening to someone else talking, crossing your feet make the blood pressure read high. Your back should be supported in the chair, feet on the ground, the arm should be supported that the blood pressure is taken in at heart level. Try that first.