looks like a birth mark? Anna has one identical to this - so does my sister and my mum... If you're at all worried call a doctor to have a look at it - my GP told me that there's no such thing as a neurotic mother, and if you're at all concerned about anything when it comes to your baby you should always get them checked out because they can be so fragile at this stage and some things need treating as soon as they occur......
Take care, I hope it's nothing to worry too much about.
[edit: seems more like an out-of-hours docto thing than an A&E visit? Call yer GP surgery and the machine should put you through to an advisor for out of hours treatment. Good luck!)