Should i be worried?

ok my daughter will be five months old on the 16th she was born 3 1/2 weeks early, the doctor said at her forth month appt. she is doing really good but i am worried. She has not rolled over yet she can't sit up on her own basically she is immobile. She hates being on her tummy and loves standing with support, she can sit by her self kinda slumped forward. She is very happy and content but doesn't seem to want to reach normal milestones. I am a first time mom so i don't really know. She has been eating baby food for two weeks because formula doesn't satisfie her, so the doc said that was okay. She has very strong leg kicks and can pull herself up with little help, I know she has the strangth to roll over ect.. but she won't. should i be worried?