First of all, big hug, sorry to hear you feeling low!
Although i agree with your firend in that it may be good for you to talk to your GP or HV about how you are feeling, i wouldnt necessarily say that they will just put you on to anti depressants. They may look at other ways to support you and monitor you. My GP was fab when i spoke to her and she pointed out that having a baby really shakes up your hormones and sometimes it can take a little while for them to stabilize.
I started showing signs of PND and anxiety when my son was 3 weeks old but it was only after a massive panic attack when he was 8 weeks old that i finally did something about it (like you, i was still functioning and getting out but something wasnt right). I needed antiDs to sort out my anxiety and am still having counselling to deal with underlying depression and it was the best thing i ever did.
Just talking to someone can help! Just remember you're not alone in how you are feeling and if it is PND, getting treatment, whatever thatmay be, will help.XxX