You asked about how long after having a baby PND can kick in? Mine started after 11 months - it was probably in the background at a low level but it was triggered, I think just after Christmas when my baby gave up breastfeeding. I treated it like a nursing strike but he just was not interested in breastfeeding and it's nearly been 40 days since then. I felt like I had lost my baby, I didn't know who this (lovely) little toddler was or how to have a relationship with him, or comfort him when he's upset, I still don't want to look at photos of him when he was a few months old, and found his first birthday really hard to deal with.
It may not be PND with you - the end of January is officially the most depressing time of year for lots of reasons, and if you're reaching a plateau of tiredness and tedium, and not seeing people, it can be grim. However, do keep an eye on it - if getting out and about and mixing with cheerful people isn't helping, you may be on the slide into PND.
I saw an excellent Health Visitor last week who, when I told her what was going on and that I thought I had PND, offered to come to our house and talk things over with me. She's a lactaction consultant so she also reassured me about breastfeeding and so on, and she's coming again this week. I think she's prepared to come every week until I'm better. I'm also seeing the GP again today (I saw her before and she advised me to go back to my HV and then come and see her again). I'm hoping not to have to go onto meds (having been on them before) but I also know that exercise is even prescribed by some GPs because of the positive effect it can have on PND.
Definitely go and see the GP though just to get the ball rolling. They may be able to help you get free counselling, or even free/reduced gym sessions, and then if things aren't getting better you'll have someone to go back to. And also try to find a good Health Visitor - tell them you think you have PND, it seems to be a magic password!