I don't think it is a cause for much concern at present as we have to give some allowance for prematurity in her case . You should continue with
Vitamin D drops as must have been prescribed . Also try to encourage the baby to lift up her head by putting attractive toys slightly over her head . The same thing you can do when she is lying on her back and you can try to encourage her to reach out at objects.
However , in your next visit to your Pediatrician you must express this concern so that specifically this aspect can be checked . It is possible to tell with reasonable certainty about normality of physical milestones at this age and stage of life