I was doing so well...

No brilliant solution to what must be a common problem. All you can do is move on. I am your height, BTW, and I can't believe I would ever weigh 120!

is there a question here?

Yes.

"Thoughts?"

I am 5'4" and my goal weight is 135 lbs. At that weight I look great and healthy. As a young adult I was about 128 and I was very thin. I have a medium frame. 120 is actually on the lean side for a medium framed woman.

Are you small framed? If so, the healthy weight is 114-127. That puts you just right in the middle.

Not trying to be a Ms. Knowitall. Just wondering if you aren't being too hard on yourself.

Cheat Days, Screw up Days, I Could Care Less Days... Those are all normal you need to forgive yourself know that its going to happen and one time a week is just fine. The bigger issue is you getting mad at yourself and feeling down or putting yourself down that's when bad things happen. So be thankful tomorrow is a new day and know that cheat days happen.

Was it a binge, where you consumed a bunch of calories in one sitting? Or did they just add up here an there over the course of the day?

I've had pretty serious problems with binging in the past and I've found two things that have helped me stop doing them. The first is that I eat at a really small deficit and take my time with losing weight. I aim for ~200 to 250 less that my estimated TDEE and eat back my exercise calories. I make it a goal to never be uncomfortably hungry or have the hunger grumps. Upping my protein and fat has helped with this. Yes, the weight loss seems slow at times, but when I restricted too much it always backfired and I'd gain more back than I lost. And I've still lost 10 lbs in the last three months even with a few days where I didn't log carefully, and a few days where I indulged in some pretty decadent dinner parties. So it's progress, and it feels sustainable.

Second, if I do eat more than I'd planned, I acknowledge it and move on. I don't even want to say I "forgive" myself because I think every once in a while it should be okay to say heck with it and sample every dessert at the pot luck. I shouldn't have to forgive myself for that. It's part of living. I just keep in mind how that choice fits into my choices the other six days of the week.

This works well for me because I think my binges were often emotional/stress related. So, the harder I was on myself, the more inclined I was to do it again.

Don't "stop yourself". Better you have the occasional overage then go to bed hungry and obsessing about what's in your fridge.

Look and see if you can recognize triggers for what caused it:

Were you out and about running errands and grabbing food on the go?
Was there a special event or lunch meeting at work?
Were you having PMS symptoms?
Were you celebrating something?
Were you not eating enough Saturday and Sunday and just broke?

If you start seeing patterns you can take corrective action instead of beating yourself up. Always checking out the menu online before going to a restaurant, making sure you carry protein/fiber bars with you when out and about, etc.

Been there, done that. Just decrease your calories by 100 for 2 weeks or something if you're worried about it.