Not a crash, but when I had brain surgery the neurosurgeon put me on gluco steroids to reduce brain swelling. That stuff gives you a massive appetite and jacks with your blood sugar. Are you on any meds?

Last time I was sidelined from ALL exercise for two months (fractured elbow) I gained probably 15# very quickly
1- Drastic decrease in daily calorie expenditure
2- Replaced muscle glycogen, which is accompanied with added water weight gain
3- Was no longer dehydrating myself on long rides, back to normal status
4- Appetite increased, did not have the post-exercise too-tired-to-eat and had more cravings for junk food not healthy stuff
5- With low exercise levels, any, uh, "irregularity" with your bowels can add a few pounds as that stuff sits in your intestines.
6- Found a new coffee drink I added to my morning routine not realizing it was 1,200 kcal per serving! Had to do a lot of internet research to find the nutritional information, it was not labeled in the store.

The 15# gain took me a long time to take off. My first priority was healing the fracture and second was regaining basic fitness from 0. Running a calorie deficit had to wait, my body already had enough stress with priorities 1 & 2.