Sounds like you are describing my lo a few weeks ago! She cut from 28/30oz a day down to 22oz a day in total and did this for 7/8 wks! She was still gaining some weight though so I wasn't concerned as the weight gain does slow down at this age! Quite a few people have had the same happen
My lo is now almost 21 weeks and the last week has gone up to 7oz bottles totally out of the blue!
I would say if she is happy and content, producing plenty of wet and some dirty nappies and putting on some weight it is fine! unless the hv says otherwise of course