That's really interesting...did you have a caesarian by chance? Some people find that they develop a uterine or pelvic infection post partum and it can cause a long term form of arthritis which can affect the pelvis and which does flare up around menstruation.
BTW I only know about this because I've got a uterine infection and have been told if it's not sorted quickly I will get this myself which will be a severe PITA with several nippers to look about!
Reiter's syndrome is what you are looking for though. Apparently it can be caused by something as simple as the flu virus as well. Luckily there are some great arthritis drugs available these days so if it is that, there is treatment.
DD is fine, she has a cold.
DS is slightly jaundiced.
I have ficking childbed fever (bloody sodding NHS hospitals, never again grrr).
BTW I thought it was normal to have bruising and a raised area following a caesarian - it turns out that's actually not all that normal and can indicate an infection, especially if the area feels hot and you have fever symptoms. Now everyone feels like crap after a cs so I'm not sure as you'd notice a mild infection, esp. as they can take up to six weeks to show up.
SPD (Ihave it too!) is made worse by carrying, lifting, general weight bearing, walking lots, standing on one leg and opening your legs so if you have done any of these more than usual then it may have flared up again, coupled with hormones. You should make an appointment with a physio.