What Causes Tiredness And Lack Of Energy While Exercising?

I'm male 26. I'm a boxer training 4 times a week and I've done so for many years. I'm in very good physical condition but around 8-10 months ago I started noticing I was getting tired and lacking energy during exercise. This has got dramatically worse to the point that 2 minutes in to my first round of pad work my muscles instantly fatigue and I have to stop. 20 seconds of rest and I'm back to normal but it keeps happening. Its so bad that I have to go down on one knee to recover and sometimes feel dizzy. I can normally do 12 three minute rounds of pad work with ease. I'm starting to really worry about it because no one seems to know what's wrong with me and its only getting worse.