Welcome to the forums. I went ahead and moved this post to where it might get some more replies, and archived the duplicate one that was at the end of an older thread. Sorry to miss your post!
There are two schools of thought on symptoms years postpartum. The most commonly accepted view says that these syndromes are often the "unmasking" of underlying conditions. We don't know about them before we get pregnant, because we're young and healthy-ish, but the strain of hosting a foreign placenta triggers the symptoms of the underlying condition. Most often this is the explanation for hypertension or autoimmune conditions like diabetes that persist postpartum. Or "the underlying chronic hypertension or diabetes is unmasked by pregnancy". As one doc puts it, pregnancy is a stress test for life - and we don't pass.
Another explanation that sometimes accompanies this one - so it could be both, argh! - says that fetal cells persisting in your body from pregnancy can modify your metabolism and immune system in ways that produce symptoms of autoimmune disease, etc. It's a fairly speculative idea but getting some traction (they talk about "microchimerism") so I thought I'd throw it out there.
there's more about this linked from my .sig file.
Since sometimes these are weird autoimmune responses and since you've already tried talking to an internist, I'm wondering about a rheumatologist. Has anyone checked your bloodwork for inflammatory responses?