Sorry for the long update with lots of details, but this stuff is totally interesting and comes to a great conclusion.
Well. Never could get a progesterone check, but I spoke with a doctor in the US (one of the top IF doctors in the world apparently!) and was able to get some more information about what may be happening. I also did a LOT of research last night and this morning. I am much less worried about things now than I was before.
Here's what I found out:
It seems that in IVF, because the follicles are damaged during egg collection, they can fail easily and not form the corpus luteum that is supposed to make progesterone. In other cases, they do form, but then fail later. This is why we're put on such high doses of progesterone- to make up for these likely failures.
Since my temp dropped so late in my cycle, obviously a corpus luteum did form and then quite possibly failed. Since my body was used to the higher level of progesterone (corpus luteum + suppositories), when the corpus luteum failed, there was a fast enough decrease in the progesterone that it took my body a moment to realize it shouldn't happen. A sudden decrease in progesterone will cause spotting, cramping, and other beginning of AF symptoms. However, and very luckily, there is a safety mechanism in place by around 5.5 weeks of pregnancy that can restore balance and save the pregnancy- the responsive placenta. By 5.5 weeks, the placenta is producing the hormones necessary to sustain the pregnancy. When it senses the decrease in production from the corpus luteum, the placenta kicks up progesterone production. In conjunction with a progesterone supplement, this IS enough of the hormone to stop the cramping and bleeding. The only catch is the pregnancy must be functioning well enough to be able to do this.
The other situation in which the corpus luteum fails is if there is something wrong with the pregnancy itself. In this case, the pregnancy actually tells the corpus luteum to shut down and the miscarriage begins. If this is the reason for the corpus luteum failure, then there
will be a miscarriage, no matter how much extra progesterone is added in, because the placenta will be unable to produce the hormones necessary to save the pregnancy as the pregnancy is already failing. Even if an increase in progesterone stops the bleeding, the pregnancy will still fail and this will be seen in future hCG tests and on ultrasounds.
Both of the above situation will lead to a low progesterone score. However, in neither situation will an increase in progesterone dosage do anything to help. In one case, the pregnancy will automatically begin to correct the situation, rendering an increase in progesterone unnecessary, and in the other case there is no way to prevent miscarriage no matter the progesterone dose- hence why a doctor would choose not to run a progesterone test. (I really wish my doctor could have just told me this!!)
As for my situation, I am VERY happy to report that I have had almost no spotting today at all, that my cramps are gone, and that my temperature has risen. I will be taking it easy for the rest of the weekend as I recover from the progesterone decrease. At this point, I am assuming I had a corpus luteum failure due to IVF follicle trauma. There may be other possible causes for sudden decreases in progesterone, but this is the reason that seems most likely and is by far the simplest explanation.
I am still not out of the woods. My hCG is much lower than one would normally expect at this point in pregnancy and the hCG stall last week still is not a good sign, but it seems I have managed to get past another hurdle in this early and complicated pregnancy- and that is definitely worth some happy excitement. I go back in on Monday for another beta test and for an ultrasound to rule out ectopic. I will let you ladies know the results as soon as I have them!
Here's a pic of my awesome temperature rise:
Hopefully all will be ok! Oh, and they aren't going to put me on BC ... which is unusual I think, as everyone I've heard on here says they're put on it first. But this clinic doesn't, they just start on your next cycle. Not sure whether to be excited or scared about the whole ivf thing! lol!
Frankly, I'm jealous! I'd much rather start stimms straightaway instead of almost four weeks of BCP. 
Sorry about the tentative PCOS diagnosis, but I don't think you should be discouraged by it. It just means they'll be watching you carefully for signs of over-responding, which isn't bad. PCOS ladies often produce tons of eggs, so FX for you!
