Regional Jet 200 ER/LR (CRJ200)
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown
ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Requested ATC Assistance / Clarification
Procedure
We were descending through 13;000 ft on the arrival when ATC told us to descend to 11;000 ft and slow to 250. We began the descent and slowed to 250. ATC then asked us what our speed was; and we replied with 250 knots. The controller said we were told to descend to 11;000 ft then slow to 250 knots. Both the Captain and I understood the clearance as descend and slow; not descend then slow. I believe the cause of this event was a very high workload due to thunderstorms ahead of us on the arrival; very busy airspace; and possible nonstandard phraseology.  We should have questioned the controller; about the descent and slow down. Unfortunately both of us thought we understood the controller's instructions. From this point on we will make sure and ask the controller for clarification if there is any question with ATC instructions.
[Report narrative contained no additional information.]
CRJ-200 flight crew reported that during descent they misunderstood instructions from ATC.