Here’s a factor you might have overlooked: what was your diet like on the 16/17/18? By any chance, were you chowing down on a lot of cold cuts, dill pickles, popcorn, fast food of any kind, etc?
Because you could have been holding onto a good bit of ‘extra’ water weigh if you’d had a high intake of salt. Then if you stopped the salt intake (maybe not deliberately, just as a sideproduct of not eating much), your body will manage to get rid of the salt AND all the extra water in just a day or two.
One time when I was in college I ate an entire jar of jumbo ‘stone’ olives – the pickled green kind. And then I started sipping the left over brine – it just tasted sooooo good for some reason. Naturally all the rest of that day I had a raging thirst, glass after glass of water with no ‘output.’
The next morning I was BLOATED. No kidding, my waist band was tight, I had to fasten my watch on another hole, I couldn’t even get my feet into my usual shoes.
I stepped on the scale: I had gained 7 pounds since the previous morning!
Horrors! But I just kept drinking water, and at some point I guess I’d added enough water to allow my kidneys to ‘waste’ it excreting the salt.
As in, the second night I had to hit the bathroom just about every hour on the hour.
The following morning my weight was back down to where it’s been before.
So, there’s a 7 pound up and down over a 48 hour period.
Similarly, I know when I go back on Atkins, I always lose 5-6 pounds in the first three days. Apparently you need more water in your system to handle a carbohydrate laden diet than one with very little, so again your system dumps the extra water.
Bottom line, I personally wouldn’t bother to see my doctor unless the trend persisted for at least a full week, maybe 10 days.