Baby Girl??

My daughter when sitting in her bouncer, car seat, pram or even my arms will pull herself so her head and shoulders are off whatever she is sitting in, i was wondering is it cos she wants to sit up?? she cant sit unattended yet and i cant sit with her holding her all day. She sit ups straight in her pram now with the straps instead of the incline position. She is only 4 months old. In the pram she doesnt lean to the side or anything but i dont know what to do with her. LOL could there be any other reason for her doing it??