I have shortness of breath and pulse rate over 100bpm when i walk, but normal sp02. what can cause this?

COPD, CHF, anemia, hyperthyroidism to mention a few of the 100's of possibilities. Would spend the $ and use uptodate to read all about it and then get back with us with more details. I recommend using all the space you can in asking questions here. Medicine is no simple.

Pulmonary & cardiac causes would be high on the list of causes. Have you had Covid in the recent past? Anemia is another fairly frequent cause of dyspnea on walking. You can have normal O2 sats while short of breath on exertion. I would start with a an evaluation by a pulmonologist or cardiologist as appropriate testing will be done.