LittleMermaidRose "She is awful to me all the time & I don't know what I done to her to deserve treatment like this!"
You have done nothing to deserve this treatment, I would imagine.
Think about it a different way, why does she do this to me, I don't warrant it so why is she doing it?
Maybe she is just a big old bully?
So why do people bully? Because they are insecure, or sociopaths, because they like seeing people squirm or unhappy, because it makes them feel good, or maybe because it makes them feel in control."
You know you could take any track with her. You could speak to her privately. "Why don' you like me, MIL?" And see what she says. I am guessing she will deny it, so you could say "Why didn't you make me lunch, when you invited me and dh to lunch? I said I was not hungry because I was embarrassed, for you, but I was hungry, and now I wonder why you did it?"
OK, I know that would be a big step, but maybe work up to it. I do hate bullies. I really think you need to get angry about this.
You said "I find it very difficult to talk to my DH about it - after all, she is his mother. I wouldn't want anyone to say anything bad about my mother." OK but she is trying to hurt you, don't you think you are worth caring about?
"When I have spoke to him about it, he always insists that it's not done on purpose" But it is!, Isn't it. you know it. So if he cannot even see it, how is he ever going to stand up for you?
" I don't know if that's just because he's used to her behaviour though, maybe he sees it as normal for her" maybe so, but at the end of the day, it doesn't matter why he thinks this. He should stick up for you. So you need to motivate him to do it, or do it yourself. Which seems easier to you?
" I didn't want to make a big deal about it in front of everyone." Why not, she didn't care that she was hurting you. Why are you worth so little that you need to go hungry. I think you and dh both need some assertiveness trainig/counselling to move beyond this.
"Plus it must be hard stuck in the middle, he won't want to cause any arguments or hard feelings in the family either." He does not need to be stuck in the middle, he can be on your side. If your comments were unreasonable, so be it. BUT wanting food at a lunch, really not unreasonable.
As someone pointed out, what if you have kids, what a field day for MIL, she will walk all over you. You can stop this now.
How long have you been married? were you treated like this in previous relationships or by your own parents? 
YOU are worth so much more.