I really don't think there's enough awareness of the post- birth hormonal crash, and how that can affect some mothers, especially if you have some susceptibility to anxiety in the first place.
Although I suffered with quite a bit of anxiety and insomnia in DS1's first year, it was nothing compared to the acute attack I suffered after DS2 was born. The second night saw me pacing the house, exhausted and in tears, but totally, totally wired and unable to go to sleep, or relax. I think that whole phase lasted about 4 weeks, but crucially I got help really fast- luckily we hadn't been discharged by the MWs, so they were able to give support.
They class it as PND, I think, but to my mind it's something a bit different from PND, which seems to come on later and be longer lasting. Apparently all the oestrogen that's been in your system throughout your pregnancy leaves your system within 24 hours- no wonder that provokes one hell of a crash in some, especially combined with exhaustion from the birth, and having a tiny fragile baby totally dependent on you!
Unfortunately there's no anti-anxiety medication they'll prescribe you if you're breastfeeding, but they did give me amitriptyline, which really helped me sleep, and that's half the battle won. Relaxation tapes helped, too, and IMO you're doing completely the right thing in getting out of the house every day- fresh air and exercise is really helpful.
I really hope you feel better soon, OP- it didn't take me long to recover, if that's any encouragement xx