Recently, I’ve been using the cross-trainer for 25 minutes twice/week.
At around the 15-20 minute mark, my feet start falling asleep. It subsides a few minutes after I get off the machine.
I spoke to my mother, and she says this happens to her too, but only in one foot.
Why is this happening? Should I be worried?
What kind of shoes are you using?
It happens in several different pairs of shoes. Unfortunately, I have absolutely no clue what sepcific kind of shoes I’m wearing except that they’re, um… sneakers.
Check how you are sitting (a cross-trainer is the excercise bike right?) You are probably compressing a nerve as it passes through your hip/upper thigh.
No, a cross-trainer is not an exercise bike. It is a standing machine. It’s sorta like… elliptical skiing.
Here’s a picture of one very similar (or the same) as the ones we have at my gym.
Here’s a slew of questions to help narrow down possible causes…
Is there any vibration in the machine while you’re using it? Wearing shoes that are laced too tight? Wearing socks that squeeze your foot too much? Color changes to your feet (especially look at the beds of your toenails)? The standard pins-n-needles feel when the feet ‘wake up’?
Of course, IANAD, etc., etc., and tossing this question at your doctor would be a good thing. (But I know it helps to have some idea of what may be going on before heading into the doc.)
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No vibration, my shoes and socks are as I normally wear them, I haven’t looked at my feet (though I will next time!).
I do have a history of bad circulation, but pins-and-needles feelings come from nerve pinchings, not blood, right?
Normally, but it’s possible that poor circulation could affect the nerves. See a doctor.
Main reason for looking at your feet is to see if they look like they should be attached to a Smurf instead of a human. It could be the angle that your ankle is at during certain parts of the movement cycle is partically blocking the blood vessels and/or nerves, and therefore switching to an exercise that didn’t make your ankle move that way would take care of the problem.
But, again, that’s something you’ll definately want to tell your doc about. “Hey, not only do my feet get numb, they look like I stole them from Smurfette!” 
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Despite all the dire warnings that “you’ll definately [sic*] want to tell your doc” about it, I wouldn’t worry about this too much, if I were you. I had the same problem with a certain model of Stairmaster. When my gym got a new version of the machine, with a slightly different geometry, the problem disappeared instantly.
My advice would be to use the machine only for as long as it’s comfortable, take a break (or go use another machine for a while), then come back to the cross-trainer for another 15 minute shot.
*The word is “definitely.”