I think at 4'10 you need to be at at least 90 lbs. (Although I've always thought ideal weight was a crock of bs, since when you're at ideal weight you're still very thin. My ideal weight is 100 lbs, and at 120, i'm still pretty thin.) BMI seems to be a better indicator.
Its not really about how you look, whether your bones are sticking out or you are amennorheic. And I'd think if you're at a weight where you're saying "well at least I still have my period, at least I don't have excess hair" etc, then you're probably walking a pretty thin line.
You're bones aren't brittle now, because you're 20. And they are at the strongest they ever will be. The concern for people who are malnourished in their 20's, (when their bones are at their "strongest" which still may be weak for someone who is 20) that you are likely to have bone problems earlier than normal, say in your 50's or around menopause.
My guess is that at 82 lbs, you are missing some essential nutrients that you need to keep your young body going and keep it preserved nicely for later!
Don't base your healthy weight on lack of physical symptoms that you would have if you were anorexic or bulemic. Base it on your health.