I would go to an internal medicine specialist or an g.i. dr. It sounds like you might be having intestinal issues and need to get diagnosed. I've also heard of people getting tumors that make their stomach protrude, but it sounds like they looked for that. Since the bloodtests are showing no baby; the ob might not know what is going on as well as someone with a different specialty. I'm not really sure that your ob could do anything else. If you got a beta test and an ultrasound; I don't think there is anything left for them to check as far as pregnancy goes.
I have a tilted uterus and have seen the sac at 5 weeks and 8 weeks from a transvaginal u/s. Very clearly. If you are breastfeeding full-time and haven't had a period, you probably haven't ovulated. I got pregnant when my DS was 8 months old. My milk basically dried up and I started having periods again. I ended up having two periods and getting pregnant.