I always found days 3-5 were the worst. .
I agree, day 3 and night 4 in particular were the worst for me, with both boys.

In the end I moved DH in to the spare room for the week (deep sleepers, sleep apnoea and co sleeping dont mix well) and co slept with DS2 for about a week, and after that it settled down a little. Mind you we were still up and down a lot but nothing like days 3-5.

As for the problem, there might not be one. Different settling techniques might work one time and not the next. DS2 would sleep between every feed one day, and be really hard to settle the next. If she has wind that wont make it easier.

Not sure about infants friend, I havent used it. Holding her upright against your body (or in an upright sling) might help?
Of if you are FF maybe there is a different teat you could use.