Hi, Petya. Same thing happened to me. I had a 4.9 feedback until I did an editing/proofreading job for one client. She wanted me to use Dropbox, but I was confused about who put what in whose account. She gave me a 3-star rating because I didn't use Dropbox, even though she praised my editng/proofreading skills. I asked her to change the rating and she refused. A real stinker!
In my opinion, if you earned $15.00 or under, I would give the client a complete refund and get rid of the bad rating.
Otherwise, keep taking jobs at lower prices (it stinks, I know). I've been doing that because every small job (where you get a good rating) brings up your rating a fraction. I'm almost at 4.4 and it's been a struggle.
Good luck and let me know how it goes. Bev L.