Hi! congrats on your acheivement so far!

You could try changing your workout routine.... ie:

change the incline on the treadmill
walk a little faster or longer
add intervals (walk fast for a 30 seconds or a minute, then slow down till you catch your breath and repeat a few times during your walk)

probablay better - do something completely different like bike or exercise video, yoga etc.... any access to another exercise machine?

and probably best - add some weighttraining... this could make the biggest difference, more muscle speeds up your metabolism and you lose more fat with more muscle. There are great videos out there like The Firm or Crunch that use weights in their program and can be aimed at beginners. Or get a book by Joyce Vedral! or another beginner's weight training book.. check your library for videos and books!

It could be that your body is used to what you are doing, and you need to spice things up a bit.

good luck!