Why does my heart race but my 02 stats stay in the low 90's?

I have IPF and am on 5 lpm liquid oxygen 24/7 and 6 lpm while doing Aqua Fit or extensive exertion. With this I have Tachycardia which the cardiologist said he cannot do anything about because it is related to my lungs. My heart is a bit enlarged but otherwise fine. I have no blockages. Given proper oxygen I have no water build up in my anckles. Why does my 02 stay in the low 90's even though my heart races and goes up to 150 bpm? I get so I cannot breathe. I cannot seem to even bring air in and I am panting and will even become nauseaus and have actually vomited. All this and my 02 hovers around 90-91. I don't get it. What is happening. I had an echo and they say I don't have pulmonary hypertension. Many friends tell me to increase my stamina by more exercise. Yet, I was declined medically supervised exercise because my heart rate was way to fast and they were afraid If I continued my heart would stop. Can I build up more stamina and correct this condition?