Lots of new mums come to my classes expressing anxiety - sometimes it's anxiety about having anxiety! There are so many emotions and hormonal changes that come with being a new parent (and being a post-natal woman) that this kind of mental stress is to be expected.
Sometimes, anxiety is triggered by new fears - worrying about your child's health or trying to make sure they're always safe. There's so much health advice to take in (around sleeping, eating, vaccinations etc) that it's no surprise many mums feel overwhelmed by fear. There's also the face that you're now responsible for a whole new person, which adds a new level of stress.
Plus, being a parent automatically makes you think about the future. (This is an easy way to slip into anxious thoughts!) You'll obviously want to provide for your child or children, and you'll start thinking about how you'll do this with caring for them and working, or moving to a bigger home, or starting them a savings account! All these extra plans, responsibilities and concerns can be overwhelming for those of us who have previously led a stress-free life.
Add to this the constraints of a new parent's lifestyle (no time to exercise, lack of sleep, bad diet and less financial means for 'treating yourself') and you'd be even more likely to develop mental health issues.
I've really enjoyed reading The Anxiety Solution by Chloe Brotheridge, and there's lots of help and links on her website which may help mums in this situation. And remember that you can always speak to your GP or Health Visitor if you think you need further support - there's a lot of focus on mums mental health now so you will find the help you need if you ask for it.