I have arthritis too and always have a flare up around 20 weeks pregnant until a couple of months after birth. The weather change really doesn't help, neither does not being able to take anti inflammatoties and pain meds. I went to my local health food shop in my last two pregnancies and they gave me some great advice along with natural remedies which can help and are safe. I can't remember what they said so will be going again this week, just as soon as I can walk there lol.
I did have problems with aching bones when I was nursing. I had a bone density scan and my density had decreased, presumably from the drain on calcium in my system. It improved again once I stopped nursing. Possibly you are having something like that happen, have you been taking prenatals with calcium in them?