I think it's because in order to increase the serotonin produced by your brain, first the drug has to shut down the production of it in order to stimulate more. I'm not a dr but I think that's how it works.
When I had hideous PND and anxiety, I went through two weeks of hell when I started my meds. I remember my mum explaining it to my niece, who didn't understand why I had moved home and was lying frightened on my mums sofa.
I will always remember my lovely niece saying, but gran, why can't they give auntie Morris some tablets that just make her better without making her worse first? Science will get there one day I'm sure, but for now we have the current generation of ssri medication.
I tell everybody, batten down the hatches and just take the drugs if the doctor recommends them. They made me better, absolutely they did. The initial side effects are a small price to pay, and over with quickly.
You don't have to feel crap. Mental illness needs treatment just like any other illness. Please get help.