That is bizarre, considering the pick would likely interfere with all signals, not just the low strings.
I'm thinking too that just the fact that you moved the amp and it seemed fine afterward, could mean something is loose or was?? Leave the back of the amp off for a while, in case it starts again, then pop in a different tube and give it a go.
Guitars: 2003 and 2004 American series strats, Squier Classic Vibe 50's Strat, Squier Deluxe Strat.
Amps: Line 6 Spider IV 120, Vox AD50VT 212, and Peavey Transtube Bandit 112.
Pedals: Digitech Bad Monkey.