My DS had quite a bad flat spot at that age, caused by always lying on that spot and always looking the same way. He was pretty determined to do this and I tried and failed to get him to turn his head the other way. His ears were quite far out of alignment. He's now almost 1y.o. and things have improved to the point that you wouldn't notice unless you were really looking at it. I think that things resolved mostly of their own accord, as he got older he would tolerate tummy time better, then he learnt to sit and grew out of napping (he only has one or two shortish naps a day, if at all). Now that he spends much less time lying on that spot things are evening out well. He sleeps in a whole variety of positions now, like any older child or adult would, though he didn't until he was more like 6 months old so this was the point where his flat spot was at its worst.
Two things I did with him that might have helped - I took him to a cranial osteopath three times, at 4 months, 5 months then 6 months old. She did gentle manipulations of his head and neck. He may have had torticollis from birth. I also bought him 2 memory foam head supports, to take the weight off his head. One is a pillow that he used during the day in his bouncy chair, and on the play gym, the other was a soft head hugger to use in his car seat. The head hugger is particularly good, he still uses it now as it's so comfy and supportive. One thing that didn't work was I bought my DS a bumbo seat to get him sitting up sooner, but he didn't like it so that was a wasted effort.
Our GP referred him to a paediatrician to have him checked out, as there are some types of head shape anomaly which need intervention such as premature bone fusing (very rare compared to the more usual pressure flat spot). She just confirmed that he had no serious problems, and there was nothing much more cold be done than we were already doing.
I'm guessing your DD's flat spot may get a little worse before it starts getting a lot better as she's still a little young for sitting etc.
Good luck!