I've hit this before. A long time ago I went from ~6-8% to a negative return in a matter of weeks. In actual fact, my return wasn't negative but LendingClub had flawed accounting that make it look so. I'll mention what I discovered in case it matches what OP did.
I bought distressed notes.. notes selling for $3 that were likely to default. It was an experiment with a limited pool of money. The default rates for standard notes did not apply here. Notes for sale tend to be near default, so they are much riskier than they appear. So my experiment buying notes on FolioFn showed high default rates, and miscalculations on Lending Club.
I later figured out what happened. When I spent $3 and had a default, I lost $3. But Lending Club used the original value of the note, and thought I lost the full $25. So every time I lost a heavily discounted note, Lending Club subtracted the full $25 from my performance. I lose a fraction of the note's value, but Lending Club subtracts the $25 someone else originally paid for it. I also found some setting that could avoid including these notes in the performance value - you might play with the performance settings to see how it changes.
OP - Did you buy notes on FolioFn? If those notes were sold at a discount, and then defaulted, it could heavily impact performance if Lending Club hasn't fixed the flaw I observed years ago.
