It depends...how fast is really fast and how low is super low? I have sinus tachycardia, which means my heart rate is almost always over 100. When i exercise it would reach 250. my blood pressure is also very low, like 80s/40s or 90s/50s all the time. I had to see a cardiologist because my low blood pressure actually started causing blackouts...i now have to take beta blockers and medicine to raise my bp.
So if your heart rate is always over 100, or it gets excessively high when you exercise I would see a cardiologist. Also if you have any fainting or dizzy spells, i would see a cardiologist about your bp.
If you don't have any symptoms like this, it may not be a problem for you at all.
How fast is "really fast"? If your resting HR is over 100 it may need treatment. My HR runs in that range at times even with treatment (stimulated by my AFib). For my condition if it goes over 130 I would call my doctor. If it goes over 150 I'd go to the ER.