Could i still be pregnant after bleeding?

My hcg isn't doubling, but it is rising.. my LMP was 12_11_11. It was not a regular period but was the result of having my IUD removed 2 days prior.
On 01_24_12 I was tested positive for pregnancy.
On 01_29_12, I had bleeding for 3 days. My hcg was 105 the day i started bleeding,  and has continued to rise, nearly doubling until this week..last hcg was 3236 on 02_24_12.
U/s shows geststional sac, which continues to grow in size. Last u/s it was 9 mm. 6 weeks?
Sac is empty..
Could it be possible that I ovulated really late and it is too soon to see anything..or perhaps, after I bled on January 29th, maybe I conceived.. again? I had unprotected sex as soon as the bleeding stopped. If thst is the case, I would be 3 weeks pregnant.
My pregnancy symptoms have faded..
Worried ..
Any answers??