Unexplained numbness!

Hi, all! I have a weird problem...I'm only 21 and supposedly healthy. So, during the last week, i noticed that when i stand up for a long time (after about one hour), especially when I'm waiting at the counter at my job, my feet will start to go all numb and tingly like they're falling asleep! i have flat feet, which cause me enough pain after standing for hours, but the numbness is something new.
Is it diabetes? Heart failure? Nerve damage? Sometimes my hands (especially the left one) will go numb, too.