It's normal for anyone, especially after exercise. Sometimes when I've done a high intensity cardio workout (i.e. running), I sometimes feel like my legs are going to give out when I get off the machine. I sit down for a few minutes and rest, and things get better.
I suppose if you're obese then it does put a lot of strain on your legs which could cause a similar situation. Keep doing the cardio and try and clean up your diet and the weight should come down which will make your legs much better.
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