My DD1 was like this at this age....and drove me insane.
I then had other issues with health from DD3 and DS2 and ignored her, and that was what did it[confused emoticon]. I am guessing that once the pressure is off them they can eat again.
My HV (luckily I did have a fab one at that point...shame about the current one>), told me then that the more fuss you make (intentional or otherwise as I certainly never thought of me as being an ogre) then they seem to be unable to swollow (and that spelling looks wrong) and so it goes on. Stress does it to me too...I just cannot eat! Feels like I have a blocked throat, and now DD1 is older she says the same.....
She is now 12, and altho she is tiny still (in age 10 clothes), she DOES eat fairly well. She also does a cycle of eat well for a month, and then not for two....which we realised when she was about 7yrs. I then fed her lots in those good months and then could slack and ignore in the 'bad' months knowing that she can eat well when she wants and is able too.
I hope this helps some.....and please don't fret. Takes a LOT for a child to eat so badly that they are deficiant in anything and as long as you continue offering and then be calm....they WILL be OK.
One other thing we do now which helps lots is to put everyhting in the middle of the table (like the waltons in fact) and let them help themselves. The only rule is that what you take, you must eat. We do ensure they take a little at a time so as not to overpower themselves and then they can always go back for more. ALL ours eat more variety now, even if the amount isn't that much more as yet.
Hope this helps