This happened to me too. From having mild patches of occasional dry elbow skin for years, that I didnt even know was psoriasis, I had a baby last january and by march it literally covered my whole body, even my finger tips (that was actually the worst). BY april, overnight it seemed, I developed a painful knee, so bad that that i thought I had somhow broken somrthing without knowing it. Then it kicked off in the other knee and thats the first time anyone ever mentioned that pregnancy induced psorriasis was quite common as was the knock on arthritis. The pain and lameness was horrendous. the skin was horrendous and as it was everywhere, creams were pretty pointless.
On line forums all spoke about sun beds helping ( and i know a million medics will tell me its the wrong type of UV to help psoriasis and that its bad for you, but I was in dire straits). I sunbedded 6 times over 2 weeks, and I can honestly say that the skin began to clear from the first visit. It was gone by the time I got my dermatology referal. It actually felt miraculous!
The knees were still bad, and I was referred to rheumatology. The first visit showed no current joint damage (I can hardly credit the pain I was in for there to be no damage). 6 months on, with the skin still clear, I have been told that I have "the good kind" of arthritis, which probably wont progress to damage and it seemed to start getting better a few weeks after the skin did. I have dropped my antiinflam/pain killer to one per day and am back to running 25 miles a week. Just left with a little stiffness and I couldnt do anything weight bearing on bent legs , like lunges or squats. I asked if there was any chance that it would just vanish as quickly as it had appeared given that the skin is clear, was told it might, but then again might not as they just dont undrstand the disease process well enough. It has never appeared anywhere other than my knees.
I have also read with interest about the possible Vit D /sunshine connection. Funnily, this march (ie exactly a year on from when it all started), I noticed some patches appearing and immediately went for another 3 sunbed sessions. As before, it vanished almost immediately. I am now wondering if it was not pregnancy related but that it was infact after a long dark winter, 5 years of no foreign holiday and several crap summers on the trot and feeling a bit fat and keeping pretty covered up, ie, the Vit D thing. It just seems funny that this year it "tried" to kick off at exactly the same point in the year which last year, just happened to be immediately post pregnancy ,so pregnancy got the blame. Who knows?
Anyway, just wanted to post a positive outcome to this illness. I was probably never as bad as you, but it felt pretty dam severe to me, and one way and another , I am almost back to normal, so there is hope.