Whoa.....you've been through so much!!!! I really don't like your doctor. With that being said, I am NOT a doctor so please take this with a grain of salt and do your own research. I just really think you need to take matters into your own hands
Did she offer a transvaginal ultrasound? Have you had one since giving birth to make sure there's not any retained tissue? It could also be used to make sure there aren't any growths in your uterus or on your ovaries, cysts that may be producing HCG, etc.
I've read a few studies that do say that there's evidence that some women do produce HCG at the time of ovulation, something like 80% of women, which makes sense because you can take HCG in medicine form to finalize the maturation of follicles and force ovulation, so your body does produce it naturally in order to create that same result. So it's possible that your body has been trying to ovulate, but I don't know how much our bodies can produce outside of pregnancy and how long it takes for that HCG to return back to zero. We just don't see it because we don't test for HCG at ovulation.
My doctor also told me that after a chemical, live birth, miscarriage, or other hormonal shifts that affect your pituitary gland, your baseline beta can raise to 4 or even 5 and it completely normal and safe until your pituitary gland returns to normal, and that's why the cutoff for betas is 5. That's why I think your doctor should have been giving you regularly scheduled repeat betas since the first time you noticed the positive results to see exactly what your body is doing over a period of time. Thank God she's doing the repeat beta today, but it needs to be repeated again and if it doesn't double or decrease, I would ask for a transvaginal ultrasound (if you've already had one then still do it again, and ask for them to check your ovaries for any growths, uterine lining too) as the next step.
If you don't see anything on the ultrasound, then, and I hate to say this, then I would ask for an CA 125 blood test as well, which is when you should worry. Maybe they did one if they did a complete CBC? Check your blood test results for it, but if it's not there, ask for another draw to include that. It sounds like your doctor isn't very knowledgeable or proactive and you'll have to take this into your own hands to get to the bottom of it.
In the case of the pituitary gland producing HCG, here's a bit of a study I found:
An elevated β-hCG in the absence of viable pregnancy can occur for multiple reasons and has a broad differential diagnosis including miscarriage, ectopic pregnancy, pituitary hCG production, trophoblastic disease and phantom hCG. Ectopic pregnancy is frequently suspected when β-hCG levels plateau or fail to double within 48 h without evidence of intrauterine pregnancy with ultrasound. When intrauterine and extrauterine pregnancy are ruled out, other causes should be investigated.
Pituitary hCG may be produced in perimenopausal or postmenopausal women. As estrogen and progesterone production decreases, releasing gonadotropin releasing hormone (GnRH) from negative feedback, luteinizing hormone (LH) and follicular stimulating hormone (FSH) rise. The β subunit gene of LH is found in a sequence of 7 hCG β subunit genes and therefore uncontrolled GnRH stimulation may lead to hCG production by pituitary gonadotrope cells (Cole, 2005).
It goes on to say that a beta level below 14 isn't worth testing for FSH anyway and if you're getting pregnant and just had a baby we are accurate to assume that your FSH isn't over 45 anyway which is another marker for that being something to worry about.
T here are almost 10 subunits of HCG, and our regular, pee sticks and doctor's regular blood testing assays don't differentiate between them. The FDA only requires that they say yes or no to the most whole form of HCG. So, although the tests can pick up that you have an elevation of HCG in your urine and blood, it cannot tell you whether it's a cancerous form or not, which is why you just need more damn testing.