I read about this online.
The Arsenic Hour
The infamous Arsenic Hour usually becomes the feature of most households with a baby between 4 and 6 weeks and can continue until she's 12 weeks, at which time it will begin to disappear.
Arsenic Hour describes a period in the early evening (sadly, it rarely is a short as an hour!) when your baby is particularly unsettled. She'll want to feed more or less constantly, and she'll only doze - usually in your arms - rather than falling into a settled sleep. There are a few theories about what actually causes this behaviour - normal development, over-stimulation during the day, over-tiredness, a fall-off of your milk supply. Whatever the reason, what is known is that after this period of fussiness, your tricky baby will suddenly calm, become tired and sleep well for the next couple of hours - a just reward!
Does it sound like your child? Hope it helps.