Need Help with Cardio

I'm having some trouble with my cardio, I can't seem to go as long as I usually do. What I do for cardio is I go on an elliptical machine, crank up the resistance, and go at about a medium pace. I can barely go up to ten minutes now, I used to be able to go twenty or thirty.

The reason I can't go much further is because I start feeling sick to my stomach. Are there any tips for me to stop feeling or power through that, or is feeling sick my body telling me that I really need to stop?

Any advice would be great, thanks.

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