baby blues can last a little longer than a couple of days. The stress of changing your life so dramatically can also leave you reeling for a while, along with lack of sleep, lots of visitors, coping with a new baby as well as other children etc etc.
Your health visitor is there to offer advice and help if you need it. Did she ever ask you questions about how you were feeling and coping? From what i remember this is usually done at about 4 weeks or so but I may be wrong.
They use a test called the Edinburgh test, theres a sample of it here
http://www.testandcalc.com/etc/tests/edin.asp
whatever you do, please make sure you answer your HV honestly, if you are feeling down then please tell her.
If it does turn out that you have PND, sounds as if you may have a very mild form then there are plenty of ways to treat it and help you, not all involving drugs. Theres counselling to and it is a well researched illness now with many sources of help and info.
Plus its more common than many mums seem to realise, so youre not alone in feeling the way that you do.