I agree with hayes, that if you show her you are upset and stressed about it, your DD will know, and may even use it to wind you up... At 18 months, there isn't much that a child can have real control over, and food intake is one of those things, so I definitely agree it's best not to show that you're worried. Also, we eat differing amounts some days to others, depending on what we're doing etc., and so do little children.
Have you tried keeping a note of everything she eats for a week, including snacks? There are probably days/meals when she eats loads and days/meals when she doesn't, and it probably evens out.
I wouldn't offer her another dish if she doesn't touch the first one, esp if you know it's something she likes. Instead, I'd just make no comment and after 10 minutes, or however long you think best, just take the plate away. Then no more food until the next snack or meal time, and if it's a snack next, don't give her a bigger snack than normal. I think eating with her is a good idea too if you can, so she can see that you eat and enjoy your food.
Especially with girls, I think it's important not to make food an issue, as food being an issue at 18 months could make food a real problem later on when DD is older.
Good luck!