This happened to me after my 4th IUI. I NEVER spot, ever. On 11dpo I had what you're describing. I was convinced it was implantation bleeding since I had never had anything like that before. Well, it ended up BFN.
But it's still so early for you! Maybe it just implanted today and it'll take a day for the levels to show up. They say the avg time for implantation is 9 days, right? So you're right on target. It's gonna be a BFP...I know it!
Keep us posted! 
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Me 31 - All tests seem fine, dx Hashimotos
DH 30 - Very Low Everything
 2 furbabies
TTC since June 2007
March '08 - First IUI w/ clomid, 
April '08 - Second IUI w/ clomid,
May '08 - Third IUI w/ clomid & metformin,
July '08 - Fourth IUI w/ clomid & metformin,
Aug '08 - Fifth & Final IUI w/ Femara,
IVF #1 October '08
9 eggs retrieved & 7 fertilized w/ ICSI
Transferred 2 blasts, none to freeze
IVF #2 in Jan '09 
11/15/08 - Started BCP's, Baby Asprin
12/15/08 - Started Lupron
12/23/08 - Baseline Ultrasound, all looks good!
12/26/08 - Started Menopur & Follistim
01/07/09 - ER, 9 Eggs Retrieved & 7 Fertilized w/ ICSI
01/10/09 - ET Scheduled for 8:00a
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