First of all, congrats on your BFP! I think it's just awful that us IVF ladies have to worry about each and every thing...every milestone... sometimes its hard just to relax and enjoy being pregnant!
I am also pregnant with twins and hit 16 weeks today. My RE took me off of the progestrone inserts at 9 weeks. I thought it was way too early but I followed doctors orders. I think my levels were lower than yours are. I did have some bleeding at 10w4d and 11w3d but they dont know what caused it.
I have heard that some doctors keep you on progestrone until 12 weeks, and think I saw someone on the September board that came off earlier than I did (see September moms to be thread). I think it was BRMOMMY2B. I have also heard that some doctors dont like keeping you on it very long. With that being said, I am not a doctor. I went through the same concerns you did at 9 weeks. Please talk to your doctor about your spotting and if you havent done so and maybe this is a good Ghost question?
Please PM me if you need to chat. I know its scary. Hang in there.