My advice would be to speak to a GP or HV about how you're feeling. PND can develop some time after your DC's arrival so it is possible.
Feeling low is one symptom - there are many symptoms: anxiety, whirring irrational thoughts, inability to sleep/eat, lack of enjoyment in life ...you don't need to present all symptoms to have PND.
The notion of wanting to 'pull yourself togerther' is also a classic - as if you're frustrated with your own inability to enjoy life. PND is an illness and not something, unfortunately, you can just shake off by willing yourself out of it.
It also is a spectrum of severity -it could be you have a mild case. I had severe PND after my DS arrived and had ADs and CBT. A lot of mums feel terrified/disappointed in themselves for 'resorting' to pills. This has a lot to do with the stigma of mental illness and most people overlook the fact that if you had any 'physical' illness you wouldn't hesitate to take medication and trust it would help you to mend.
It's not for everyone so chat with your GP and consider ways you can improve your mood - getting physical support (being a mum is exhausting by nature), taking exercise, eating healthily, relaxing and making time to do things you enjoy. Therapy is something you might consider - it helped me immeasurably and gave me the chance to talk about things to an impartial stranger who would listen without judgement.
Hope you feel better soon, look after yourself :-)