I recently brought my first bonsai, it is a tree of a thousan stars bonsai i have had it for just under a week having it potted professionally when i got it. In the last few day quite a few of its leave have turned yellow with a dark spot on the tip of the leaf and have started to drop off!!! Is this normal or is it dieing I have no idea and I really love my little tree please help me!

My two cents: I had a serissa that did pretty much the same thing, though it wasn't after a repotting.  I had picked it up from a boot sale...what I found is that the potting soil was (or had become) a very closed mix...meaning the water didn't flow through it and drain out.  If you used a premium general potting mix or (even worse) a potting mix with some planting compost mixed in, it may be too fine without sufficient grit.  Like Chris said, the roots are prone to rot, which is what was happening in my case.  If it's just been repotted, you probably don't want to stress it again by another repotting if in fact your mix is compact.  I guess you could use a kebab stick and gently aerate the soil to make sure that when you do water it, it's not retaining too much at one time.  I can't say for sure whether this is what's going on with yours, but mine had the same symptoms and soil with insufficient grit/gravel was to blame.  You could also check to see if the mesh over the holes at the bottom of the pot have clogged up, preventing the water from draining. (Chris-- feel free to correct me)
Good luck with it....