What is going on?!?!

I miscarried at 5weeks 1day on July 12, 2016. On that day, my hcg was at 30 and my urine test was negative. The Dr. Seems confident that I was miscarrying naturally and did not do a sonogram or any further testing. I went back to retest my hcg on July 19, 2016 (exactly one week after miscarriage), and my hcg levels are now 336. The entire time, I have been getting positive HPTs. What the heck is going on? Why is my hcg level going UP?