I was taking in about 1800 calories and lost about 5lbs in 3weeks. I wanted to lose a bit quicker so I changed my intake to about 1300-1500 and I actually gained almost 2lbs in just a few days.

I am a part-time nurser, not sure if this makes a difference or not.

Any advice or tips? Also, I wasn't working out while taking in the 1800 but since taking in the smaller amount of calories I've been walking a mile a day and still gained.

Your calorie intake was too low which caused your body to go into starvaton mode. Here is a link to figure out your calories ( it is only an estimate)
http://nutrition.about.com/library/b...tion_guide.htm

Not necessarily. While that calorie range is too low for a nursing mother, that doesn't mean her body went into starvation mode. There could be many reasons for the gain. I would associate the starvation mode however with a drastic cut in calories to say under 1200. I'd feel the likely culprit was adding exercise, therefore building muscle, which weighs more than fat. This happens just about anytime someone first starts exercising.

You don't build muscle in just "a few days." Many people do gain a bit when they first start exercising; it's usually either b/c their appetite increases or b/c the newly-stressed muscles hold on to some water weight.

Yep, any big changes over a couple days sounds like water to me. I'd just keep plugging away and see what happens over the next couple weeks. 1300-1500 cals isn't bad for a light nurser.