Firsttimemum,
I use EURAX cream (they also do it in lotion form as well) for my family. I'v read the back and it says for children under 3 consult your gp and then to apply it only once. It lasts up to 10 hours and it is truely brilliant. Piriton syrup will also help but please check that you can take them together. You could also try traditional Calamine lotion.
Curiosity is right thou, check out babies diet and check to see if you have changed any household items such as washing powder, conditioner, soap, bubble bath, baby oil, nappies etc anything that may have come into contact with babies skin or anything she may have eaten.
Is baby crawling or moving alone? If so, check you flooring, anything like this could inflame Excema.
Good luck and try a cool bath before bedtime then rub in any cream/lotion etc directly after - warm skin helps dissolve the lotion in the skin faster.
Flower xx