Need advice - baby is 4 months old, can't hold her head up

Hello, my little one is Just coming up to 4 months old. She was born 3 weeks early only weighing 4 and a half pounds..  She still can't hold her head up when doing tummy time and just screams at me when I try and help her. Does anyone have any advice? 

Thankyou in advance 🙈 x