I slept on my tummy until about 20 weeks, if it's comfortable for you then your fine and it won't hurt the baby(: you'll get uncomfortable before it is even close to harming the baby!
11 weeks it's still safe to sleep on tummy as long as you're not uncomfortable. And to buy things I think it's way too early I would say after 7 months... just to be on the safe side
I started buying things from about 11 weeks but it's more fun when you know the sex. To be on the safe side it's best to lay on you side but Coz you don't have a bump I don't think you will be causing any damage x