It is not at all uncommon for an autoimmune condition to be triggered after childbirth. During pregnancy, your immune system "downgrades" its reactions to foreign entities so that you can keep the fetus. After childbirth, the immune system revs up again and sometimes over-revs to the point that it views your own body parts as foreign entities. It depends upon your genes, which body parts/systems are impacted. If you have RA, then you very likely have an HLA DR4 gene.
One other thing you should be tested for is celiac/gluten intolerance. The research keeps pointing to this being the root of many autoimmune conditions. It's an intolerance to wheat gluten, thus your immune system generates antibodies to the gluten - these antibodies are believed to trigger the other autoimmune diseases (it's fairly complex, but has to do with impacting your innate immune response, elevated cytokines/inflammation, increased zonulin, etc.). And the genes for celiac correlate with the RA DR4. You can search in pubmed and find LOTS of medical research on the correlation. And, you can find lots of information on folks whose RA went into remission when they went on a GF diet. Also, nightshades (tomatoes, potatoes, eggplant and green peppers) can aggravate RA.
So, I would strongly recommend you get your doctor to screen you for celiac (You can find lots of medical research as to why this is very common with RA or I'd be happy to send you something you can print out and take to the doctor) or better yet, you can get tested on your own by ordering a test kit from this company: www.enterolab.com
This test is far superior to the blood tests your doctor might run for several reasons:
1) it's more accurate (it's 100% accurate if you have full blown/end stage celiac, but also catches the inbetween stages of gluten intolerance)
2) most doctors don't order the right tests for celiac - it requires 4 CPT codes to get the complete test run with both Quest and Labcorp
3) Few doctors will even consider the possibility of celiac because they aren't trained to look for it. It is the most commonly undiagnosed disorder in this country right now - there are 2+ million people that have it, that don't know it. The average time to diagnosis for most people is over 7 years.
4) In the end, you'll save ALOT of money - the only treatments they have for RA all deal with symptoms and some of the primary meds they've used have caused death (i.e., Vioxx).
5) Per the 7+ years to diagnosis, the sooner you catch this disorder the better off you will be healthwise in the long term.