I was prescribed Mirtazapine years ago. It's an anti-depressant and I was certainly warned that it can take some time to work and I could start off by feeling worse.
It's also used as an aid to sleep, which was one of the main reasons I was prescribed it. Additionally, the way it works often causes weight gain.
Stopping them after five days won't have given them any chance to work, although it's not surprising that there's been a rebound. As others have said, it won't take more than a few days to get them out of your system.
I took them for nearly a year, but decided they weren't working as an anti-depressant. They didn't help much with my sleep either, so with my GP's support, I went cold turkey (against usual advice). Apart from feeling less dopey, I didn't notice much difference, so I guess they just don't work for me.