Thank you QP, UUM, JH and Jasper for your replies!
JH, I hadn't thought about the fact that it gets worse in winter, but I am pretty sure it does, or at least more noticeable. Vitamins just to be on the safe side is a really good idea.
UUM and QP - I will try and observe if it is worse when she is tired, very good tip. I suspect that it is - although she does seem to get a good night's sleep, I know that she does have a kind of adenoidal breathing thing which makes her breathe very noisily and awkwardly while asleep.
Jasper, it could be the pale skin that makes it noticeable. When I think about other children where I have noticed a kind of redness around their eyes, they have tended to have red hair and/or pale skin. It could just be the transparency of the skin that makes it prominent. That fits with winter as well when the skin pigment is slightly lighter.
All very good suggestions. I think I will give the vits, observe when it gets worse, and as long as she is not ill, I will try not to worry.
I don't know if any of you felt this, but when I am out with her, I always think she looks really ill and I suppose I worry that other parents think I am not feeding her or neglecting her in some way.