Hi there!
I agree 100% with the others; don't self-medicate. PIO shots give you a big dose of progesterone that wears off during the following 24hrs... then your body has nothing for 48hrs and then another big shot. Your placenta is producing all the hormones your baby needs now, and it's been producing them since week 10 or so. I agree with Silver, you should call your doctor and have him/her explain why it's a good idea to stop now. But there's a reason why almost all the doctors we hear about here make us stop cold turkey style.
One thing though. My doctor warned me that minor brownish discharge is NORMAL after you stop artificial progesterone. Your baby isn't bleeding; it will most likely be your cervix. I had it some 4-5 days after I stopped the vaginal inserts. Lasted around 48 hours and it was gone. It wasn't scary because I knew it was going to happen. And believe me, when you see what your baby looks like at 12-13 weeks, you realize that a small amount of blood in your panty liner makes absolutely no difference for him/her!
Either way, trust me, you'll feel MUCH BETTER after you stop taking meds. It's worth it! Like the others said, you made it through the first trimester, your miscarriage risks are lower than 1%, enjoy your pregnancy dear, you're going to be a mommy!

:D:D
Good luck,
Sophie