Why am I putting on weight?

I agree with cheryl. 1000 cals isnt enough. try sticking with the target amount recommended by mfp. it will never recommend less than 1200 net cals to you.

Try eating peanut butter, nuts, or use oils when cooking.

You need to eat more. Your body is in starvation mode. Eat more and your body will begin burning calories from your body's fat stored instead of from your muscle which it is doing now.

I hate to open up this can of worms, but I also hate seeing incorrect information posted. Her body is NOT in starvation mode in eight days. I agree that she should be eating more than 1,000 calories a day but it is certainly not starvation mode.

I made the same mistake the first two weeks that i started. I limited to 1000 cals/day. Not only was i in starvation mode but i couldn't exercise to my full potential because my body was too weak.
I did some research about how all that works, it helped me understand this whole fat loss process better.

I've been limiting myself to 1000 calories a day for eight days now, and in the first five days I lost six pounds. Weight was simply dropping off every day. But the last two days I've put on weight. I've put on three pounds in two days! Why am I putting on weight when I'm still eating only 1000 calories?

In the first FIVE days you lost SIX pounds of WATER.....and then your body wanted to balance itself and kept the water back....
It's to early to yell starvation, just keep on doing what you are doing, except for jumping on the scale every day, and then expecting miracle weight loss....
You will lose more, but a heck of a lot slower than six pounds in five days.....

Spread your calories about through the day....If you are eating too much, too early, then you won't be hungry later on...

Try adding peanut butter, nuts and cheese. Eat full fat yogurt instead of light. I had to increase my calories to keep losing, and it helped. If increasing the amought of food is hard just try and find things that are higher in calories.

Can't be starvation mode.

But metabolism can easily slow down in 3 days if severely undercut from what was being done previously.

Not being hungry to even eat the 1000 shows that has happened.

Exercise not even mentioned in the comments, but could be making things even worse.

And one day of eating more carbs in that 1000 calories could have topped off glucose stores that are severely low. In that low state, that means muscle is being broken down to provide amino acids for conversion to glucose, what your brain requires and the body will provide, if nothing in the tank.

So you probably had a day of more carbs, and each g of carbs stored has 2.7 g of water.
Over course of several days, if you topped off the 1500 calorie easily lacking from the tank, you could add 3.3 lbs of weight.

Not fat, but good, needed weight.

So eat more, start with dense stuff.

Or stay on this route, keep breaking down muscle, and then see how fun it is.
You could accelerate that fun by keeping your carbs reduced, throw in some long walking, and really feel the muscle melt away.
And since it only takes a 600 cal deficit to burn a pound of muscle, you could actually lose weight much easier.

And yes, i'm being sarcastic, but totally honest as to what you could do to your body. Reread that if you don't get it.
Research it if you don't understand it or believe it, basic physiology - that's your homework.