help please

my 5 month old had a visit at WIC today. where they checked his weight and height. I changed his diaper after that visit and he did fine. i dropped him off at daycare and when i brought him home to change him, he screamed when i straightened his legs out. he is fine if they are pulled up. when i gave him his bath(which he normally LOVES)he refused to straighten his legs out. and again tonight before i changed him before bed, he screamed when his legs were straight. but if he curls them up he is fine. anyone know what this could be based on experience? does it require an ER visit ? was WIC or daycare too rough with him?