Help... need some insights

I know u mamas must be sick of me and my paranoid questions by now... I promise this is the last one... I am back on medication so let's hope...

So I massage my girl everyday... for the past few days when I try to bend her legs inwards as an exercise she stiffens it like a rock... if u smile and talk she will let it loose again... happens again and again... also sometimes while I am trying to make her sit she will stiffen it and try to stand... she is 5 months old..

Do any other babies do that? Do ur babies intentionally stiffen their hands or legs too??? Do you think that's normal and that she has just realised she can stiffen her legs??