Why does my heart rate increase so much when i stand? and my knees get tired quite easily and sometimes i feel like im going to faint? my rhr is 67 and when i stand it goes to 100-115ish. i have healthy mobility habits and dont drink or smoke?

POTS or postural tachycardia syndrome is a condition where blood pressure drops precipitously when you go from sitting to standing. The heart tries to compensate by increasing its rate to raise the blood pressure. This condition is more common in young females than young males. As the years go on symptoms tend to lessen as the autonomic nervous system strengthens.Increase salt&water intake helps

Your description suggests orthostatic tachycardia. My suggestion is that you see your physician for an evaluation. I am asking you to do this since you have accompanying symptoms( you feel like you are going to faint). Your doctor will be able to examine you, record your blood pressure on standing and make further recommendations. Please do this without delay. Good luck.