Why am i losing weight so slowly?

I'm 5'3 and I used to weigh 185. The first 23lbs came off easily (3lbs p/week and this was done under the supervision of a nutrionist) but ever since 162 i've struggled hard to lose weight. I'm at 149 now and it has taken a week to lose just ONE pound! Why is this? When i hit that plateau at 162 i changed my exercise and meal plans, but it hasnt been long enough for me to make another change. What's happening?