Whenever she's on her back or sitting up my DD is constantly slamming her feet (heels) on the floor. Sometimes her arms too. It's like she's having a fit or something but she's perfectly awake! It looks so painful because she does it so hard (she's got freakishly strong legs) so even on carpet/mat it must hurt. My DH put insulation pipe all round her changing table because she's a bit long for the mat and she was doing it against the wood 
She loves swimming and baths and does the same on the water so is it like she doesn't realise she's on dry land or something? 
Is it normal? other babies I know don't seem to do it.