"areet mate... i have just been in the same situation.... i got my ear stretched professionally to 6mm and that was fine... yesterday i popped an 8mm in there myself (the swelling is a pain!!!) its very tender, quite sore and I'm getting a lot of puss in it... here is why lol
when you have a piercing done after the usual 6-10 weeks waiting period scar tissue forms. now scar tissue is a knob! lol its a lot tougher than skin and flesh and when you have flesh tunnel LOADS of the evil stuff forms...
once you have had your ear stretched once it requires a little more force to get a bigger one in...
my suggestion: don't take a shower or bath as this will not soften the scar tissue, instead... take you old 6mm plug and put some Vaseline on it then push it through your ear... once your ear is nice and lubed up take your 8mm and force it through as hard as you can... you will hear a "pop" and feel a tear, this is the scar tissue tearing and it hurts like a mother bi*ch! clean with a salt water solution twice a day for 2 weeks and HAPPY DAYS"
a little improvement -- Use Ibuprofen Gel instead of Vaseline. It works just as good as a lube to help it slide in, but it makes the surrounding area go slightly numb, so it'll loosen up a little to help it stretch easier, and of course there will be less pain since it'll be a bit numb.
I'm up to 18mm now mate and what i found was after you leave your 6mm heal for the 2 weeks, once healed take em out and put some PVC tape round em, put em back in (after lubing of course.). Leave em for another week or so then when you go to put your 8mm in your ear has stretched slightly so it takes less effort to get the next size in. Remember lube and cleaning are the two most important things when stretching your ears. Personally i use a water based lube like KY and i find it easier to use than vaseline but this really is just personal choice.