It really Hurts

Are you using any lubrication to make penetration easier for you due to the increase in his size for you?  That will make it a little less painful and easier to accommodate. 

Yes, unfortunately it will take a bit of time for your vagina to stretch to fit him comfortably.  It will but just give it some time you have only been with him for one week.  It's kind of like when you were a virgin and you stretched as you were tight, now you have a bigger size so kind of the same way as that.  When and if you break up and next bf is smaller, remember you will have to adjust to his size again as well, you may not feel like you enjoy sex as much because you won't 'feel' as much due to you being stretched bigger than the average male.

Hope this helps

Most women's vaginas should be able to take most sized penises. BUT, it is really important to be aroused and well lubricated. What has probably happened is that after the first time you learnt to expect pain when you have sex so you get tense when you try, and then its hurts. Get him to take things really slowly. First have lots if foreplay, with him fingering you, playing with.your breasts, kissing, cuddling, stroking, going down on you etc. When, and only when, you are very aroused and wet should you try intercourse. Make him be very gentle at first and take it slowly so you don't tense up. It might help to be in a position where you can control things, like you on top. Over time you should get used to his size. If that doesnt help then see your doctor. It could be something called vaginismus which causes painful intercourse. But because you haven't had this problem before try the above suggestions first. All the best.