Think of it this way. You have to eat 3500 calories more than your body needs in order to gain one pound of fat. (It goes the other way, too: you have to eat 3500 calories less than your body needs to lose a pound of fat.) Nine pounds equals 31,500 calories. To ground that number in reality, a quarter pounder at McDonald's is 510 calories, so 31,500 calories is about 61 of those hamburgers. Did you eat 61 hamburgers in the last day? Of course you didn't! This 9 pound "gain" you just saw is almost certainly related to how much water your body is holding onto. The great thing about water weight is that it's temporary: eventually (usually within a few days), your body will let it go.
By the same merit, this is also the reason why everyone is saying you probably lost a lot of water in your first week. 15 pounds is
a lot to lose in just one week. 15 pounds is 52,500 calories less than what your body needed, which is basically physically impossible.
Please don't get down on yourself. All of us started by shedding water in the first week or two. It's very normal. Just know that among those pounds of water were also a couple pounds of actual fat. You ARE making progress!!
Let me tell you a story. In my first month, before I knew any of this and assumed weight loss was linear, I lost pretty good amounts of weight for the first couple weeks. I was on a roll and feeling good. Then, the third week, I suddenly stopped losing anything, and started seeing gains. I felt much the same way you did: horrified. After doing research online, I came to realize that there was a possibility that it had to do with my cycle. Sure enough, it turned out to be ovulation water weight, and it went whooshing away the next week.
The moral of the story is, the number on that scale is not just fat. It's bones, organs, blood, water, everything. The scale is a dirty rotten liar that actively tries to destroy self esteem, so don't listen to it. You're naturally going to pick up water one or two weeks a month just because of your cycle, and it's not because of anything you did. If you're still working and on plan, you're still losing fat, even if the scale doesn't say you are.
As those above me have noted, 15 pounds is a lot, and it's possible that your body is stabilizing. It's also possible that you're ovulating, like I was. If you've started exercising recently, your body could be holding onto water to repair itself. You could even be coming down with something. There's a lot of reasons why you retain water, and none of them (except maybe eating salty food) is ever really because of something you did. Just keep working and believing, don't give up, and it'll all whoosh away again plenty soon!
If you want to help your body flush that water away, try drinking more of it. When your body gets lots of water, it feels less of a need to hold onto it. It's not a precious resource to be hoarded anymore; it knows it'll be fed more of it regularly, so it doesn't need to hold onto its stores. Make sure you're getting at least eight 8oz cups of water a day, but drinking more than that only helps! I don't know how IP works, but if it lets you, you could try eating foods high in potassium, too. The way I understand it, sodium (salt) helps your body store water, and potassium helps flush it out. Bananas, potatoes, mushrooms, fish, squash, yogurt, avocados, etc. are all good for potassium.
I also definitely recommend taking measurements every week in addition to weighing yourself. The scale is a dirty rotten liar, but the tape measure is much more honest. Sometimes the scale will claim you've gained, but the tape measure will say you've lost half an inch off your belly. I'm inclined to believe the tape measure.
Sorry for the book response, but
please don't beat up on yourself, and
please keep eating! It's okay! This is a very normal phase of weight loss, your body dropping water and bouncing around a little bit. Remember, weight loss is never linear, and it bounces around a lot because of our cycles. It also varies by up to as many as 5 pounds at different times of the same day! (Incidentally, the best time of day to weigh is right after you wake up, after a bathroom visit.
) Keep records, and if you're trending down over several weeks or months, you know you're making definite progress. Daily weight tells us surprisingly little about our actual weight.
Keep on moving, girl. You've got this!