I wouldn't worry about slow down, it's entirely normal and can be rectified quite easily if you change things a little.
For example:
When I did my first diet 2 years ago I lost the first 1.5 stones in about 7 weeks and then found the pounds took a lot longer to shift so I just changed my diet slightly.
I found that as I got used to getting more excersise my body was becomming far too used to the type of excerise I was doing so I added a little bit more variety to my plan (which stamped out the repetetiveness that had creeped in) and I soon saw an improvement.
Good luck with your weight loss!