Variance - Convergence of winrates - Am i a getting worse?

[Rotating Rabbit]

Yeah so i've been playing 2/4nl for a few years with a lifetime ptbb/100 of 8 (this is mainly a hobby evenings/weekends). I had a month off last month (travelling) and this month i've played around 20k hands so far with a pitiful 3ptbb/100 winrate. This kinda disturbs me. How many more hands would I need to play to be reasonably sure that my skill had deteiorated relative to the competition?