Welcome to the boards, Shannon.
I'm really sorry to hear that that person told you anything about what they think you should do when it comes to your body. It's not their place to do that: their body is the only one they need to be making these kinds of decisions or statements about. I'm doubly sorry this hit you so hard.
Dieting, as most people think of it, actually is rarely healthy, and isn't something anyone should be doing anymore unless supervised by a health professional. We have a lot of data on dieting we didn't used to have that shows it more often results in negative health impacts (and also rarely in sustained weight loss when that is actually advised for someone) than positive ones. To boot, even the BMI is something a lot of people, again, through study, know is flawed. Your weight is likely a perfectly healthy weight if a healthcare provider, evaluating you properly and with care, and current education about health and size, hasn't suggested it isn't and is something you need to change for your health.
But the real thing is that even if you did lose weight? That probably wouldn't change how you feel. What you're really voicing here is about body image, and people of every shape and size can struggle with that, even people who meet a given beauty ideal to the letter. People in the headspace you're in who even *do* change the thing they say is their problem often just find something else after that is The Big Problem, or emotionally move the goalposts, deciding that however thin that is still isn't thin enough.
In other words, this probably isn't really about how your body is, it's about how you're seeing it and thinking about it, something this person at work sure didn't help you out with. It probably isn't so much about your body as it is about your mind, long story short.
There are different ways to approach a negative body image -- presuming that's all that's going on here, and not a bigger issue like an eating disorder -- and work to improve it. You have options. Would you like to talk about those?
If so, can you fill me in on if these feelings about your body are brand new to you, or if you have had them before this comment made at work? If they have been around for a while, can you fill me in a bit on your history with them? When did they start? What do you feel like really set them in motion? Too, have you had anything else in your life that might have creates a struggle with how you see yourself and feel in the world, like any kind of past abuse, neglect or harassment?
I also noticed your other thread about having sex with a partner. Is that something new for you? I ask because sometimes being new to sexual relationships -- or even when it's not new, certain dynamics in them and parts of them -- can certainly be something that can bring up feelings of insecurity about our bodies or make negative feelings we already had more intense.
You can also take a look at some of the content we have about all of this on the site, like
The Scarleteen Do-It, or this one --
http://www.scarleteen.com/blog/heather_ ... ofrightnow -- and content you can find at the body image tag on the site:
http://www.scarleteen.com/tags/body_image
I do want to add that there are fat people in the world, and some of them are right here, using and at this same service. Doesn't sound like you're one of those people, but it would certainly be okay if you were, just like it's okay if you aren't. If you ever are fat, that'll be okay: it's not shameful or unacceptable to be a fat person, because bodies come in a wide array of sizes and shapes. There's truly nothing wrong with being fat or being a fat person except that we've got a crummy, often uneducated, ignorant world that can often suggest that there is. So, it's important in discussions here we make sure we're all trying our best to talk in ways, even when we're grappling with rough feelings, that make this place feel safe and sound for everyone.
(Fair warning, I may not myself be back here today and potentially longer, as we're just about to get hit with a very bad windstorm where I live and may be without power for a while if that happens.)