I've eaten soup and shot up 3 pounds for days. One stupid meal with a little too much salt can trick the scale because you're retaining water. It doesn't have anything to do with your actual progress.
I've also had the experience of eating way too few calories and putting my body into "starvation mode." That stuff is NOT a myth. You can cut too many calories at once and scare the crap out of your body and make it cling to fat because it doesn't know when the next meal is coming. You're gonna have to play around with things and figure out what works for you.
But please don't give up. I often feel like what's the point and sometimes I have to focus on my health and the way I feel rather than the numbers because it's too frustrating. I've even taken months off and just focused on not gaining any weight and it's actually worked out pretty well. I'll lose 20-30 lbs, maintain that for months and then get amped up to lose some more. It kind of takes off some of the pressure and stress of CONSTANTLY weighing myself and exercising like a fiend. I've read that in the long run people who do that are more likely to keep off the weight.
Anyway, hang in there and believe in yourself. I often catch myself thinking "what's the point? I CAN'T lose weight" and then I remember one of my favorite quotes by van Gogh:
"If you hear a voice within you say 'you cannot paint,' then by all means paint and that voice will be silenced."
So tell that voice in your head to go to **** because you got this.