Cryptic pregnancy?

Has anyone had experience with a cryptic pregnancy? I think I may be having one. I haven't had a period in almost 3 months (I was very regular before and after giving birth) and I'm experiencing the same symptoms I had with my first pregnancy. Headache, back pain, nausea, food aversion, acne and I've even gained around 10lbs. This entire time all hpts have been BFN and I feel crazy. If I'm calculating the dates right I would be about 12 weeks along right now. I keep waiting for the bfp because I know my body, and my body feels pregnant. It's not like we are ttc, my LO is only 6 months old. I've heard sometimes if you conceive soon after you've given birth the hcg may not be very strong? When I lay on my stomach I have actually felt a little "popcorn flutter" just like I did with my daughter. My doctor doesn't believe me at all and won't do a sonogram or ultrasound. I'm just absolutely confused. Anyone with experience of low hcg, or cryptic pregnancy HELP PLEASE!