timestamp,stockname,total volume traded

11:00,A,100
11:00,B,500
11:01,A,150
11:01,B,600
11:02,A,200
11:02,B,650

timestamp, stock,total volume, change in volume
11:00,A,100,NaN
11:00,B,500,NAN
11:01,A,150,50
11:01,B,600,100
11:02,A,200,50
11:03,B,650,50

df['ChangeVol'] = df['TotalVol'] - df['TotalVol'].shift(1)