My daughter was 8 weeks early and is 5 months old now. She used to cry as soon as I put her on her tummy, but this is getting much better, she lasts about 5-10 minutes now. Have you tried putting a a little pillow under her chest when you give her tummy time? I found it helped Morgana a lot to be slightly propped up. She started twisting to one side when she lies on her back, so I assume she is getting ready to roll over in the next month or so. Remember, you can't measure your lo against other, non premature babies of her age or you will get frustrated.