It seems like overnight your co-worker lost 40 pounds and looks fabulous. So, you're probably a bit jealous of your co-worker's weight loss, because that's just human nature. When you are jealous over someone's weight loss, the first thing people say is it's because you are overweight yourself. That's not always accurate.
When someone in our life improves in any way, whether we like it or not, we may have a twinge of jealousy. It's natural and it doesn't mean we wish them ill-will at all. Hopefully your co-worker isn't rubbing her weight loss in your face, but she may appear to have a different attitude. Of course she does. She's proud of herself, she worked hard to lose the weight and she physically feels better.
Let's say you actually are overweight and maybe when you see your co-worker it's a reminder that you have gained weight and haven't taken action to lose it. You can turn that twinge of jealousy around to your benefit. Talk to her and find out how she lost the weight and could she share her secrets with you. You could go as far as asking her to be your weight loss co-dieter and work together as a team. This way she can lose more weight if needed, or at least maintain her current weight loss and you will improve yourself and begin to feel on top of the world.
Start a walking schedule that benefits the two of you and you may even want to invite other co-worker's into your diet team. Once you begin to lose some weight you won't be jealous of your co-worker's weight loss, because you are beginning to look and feel fabulous too. If by chance you really don't care for this co-worker at all, that's okay. You can get someone else to join in with you and if there isn't anyone else, get focused and decide on the weight loss method you are going to take and begin taking baby steps. As the weight comes off, the easier it is to continue until you've reached your weight loss goal.