It's a build up of lactic acid...don't worry it means you're pushing yourself to the right levels. As you get better it will take more and more effort to reach that point and after a while you can easily maintain without getting to that point. It may not seem harder but I'd say they way you're doing your exercises (super setting?) isn't allowing enough time for the lactic acid to clear. So at the gym you may be doing harder exercise but more time between sets allows thenlactic to clear.
That's why hard core gyms have puke buckets.
Keep up the good work and keep the window open.