You didn't say what you've been using for birth control, or if you are using birth control.
If the blood test came up negative, how close was it to your TOM? Some women don't produce hcg in detectable amounts right away.
I hope the ultrasound will help confirm things. I'm not sure if fibroids or cysts can cause those symptoms, but it seems like that could be a possibility.
It could be something like mono, if your thyroid levels checked out okay, and other hormone levels are all in check, it does seem very odd.
How old is your daughter? If she's young and you nurse, this could also be your body adjusting to your hormones shifting from breastfeeding. If you've recently weaned her, that could also be a factor. A close friend of mine had a wonderful first year with her son, but when he weaned himself at a year old, she ended up developing a pretty bad case of PPD... and had some symptoms like yours, but not the constant nausea you describe.
I hope you are able to figure out what is going on. I hope it's an easy remedy!