From your writing, I am guessing that you are not from the US and that English is not your first language. It is difficult to learn about new cultures and the unique things about each one. I speak a second language (Spanish), but not entirely fluently, and I know it is easy to make mistakes. I also know that things I could say in one Spanish-speaking country don't mean the same in many other Spanish-speaking countries. Usually, people are very kind and try to be understanding of errors I may make.
As you can see from the responses below, most people on Mamapedia are being kind, although there are one or two who decided to attack you and even ask whether this is a real question. I took a moment to look at your previous questions, and I saw that, while your English is good, it's still hard for you to communicate exactly what you mean.
So, in the US, as others have said, it's considered rude to comment on people's weight, even if you know them well. I'm not sure if you even know that "plump" means "fat", or that many Americans struggle with being overweight. When you tried to correct it, by saying she looked "tired", she might have still had hurt feelings, and also she might have taken that to mean she didn't look healthy. So people don't like to be told there is anything wrong with how they look. We also don't do a lot of personal conversation with people we have just met.
We also don't usually buy gifts for people we don't know well or who are work colleagues, so your very nice gesture of buying a wedding gift may have been viewed by her as being way too personal for a business relationship.
I think you should just apologize and explain that you meant no offense, but that you didn't know what the word "plump" meant, and that you didn't mean to insult her at all. Tell her that you struggle with small talk because of your language and because of your personality, that you were trying to be friendly but you feel a little awkward about it. Say that you hope you can move forward with a business relationship and casual, non-personal conversations.
Now, let me also say that it was very rude of this woman to call her friends and tell them about you when you could overhear the entire conversation from your desk. So you might say that you know you hurt her feelings because you couldn't help overhearing her talk about how upset she was.
From now on, concentrate on doing a terrific job, and on just making small talk about things that are not personal - the weather, how the local sports team did in their last game, you hope to get to the lake or the beach next weekend and what does the other person have planned, what movie or restaurant you visited and does the other person have any recommendations, etc. If this woman cannot forgive you, then she's not a very nice person and you have no need to continue to try to be her friend.
Good luck.