Does she pull on the nipple? Usually this encourages another letdown. The fussing can be not enough milk at that moment in time (a few moments later all is OK, impatient little people), or too much in belly with air as well.
If you think the initial letdown is too quick (and they do get better at coping with it), try expressing off a little before the feed, and feed in football hold (under armpit), or any position that has the baby more "upright", you wouldn't want to feed lying down beside her, when letdown will gather in her mouth and choke her a bit.
If you think she's not full and is trying to encourage letdown, she will do this by her own suckling/pulling as long as it isn't painful to you or damaging nipples, supply will increase.
The shaking head thing when off the nipple is her looking for it (even if it's there!).
Overall, unless it is causing you pain when she latches and unlatches, or she is getting into such a state she won't reattach, it's just a phase that she will work through - either by coping with the letdown better, or getting the supply to increase a bit.
But don't feed her with a bottle (even expressed milk) as she is still obviously learning to breastfeed, and if she gets milk from the bottle with minimal effort then she won't put the "work" in to make breastfeeding work for her. Once she is an easy feeder, then introduce the bottle for occasional feeds.
Some say the dummy can be detrimental to learning feeders, mine were all OK as they wanted a comfort suck and if I left them on the breast they got overfull and were sick, but another thing to consider is whether she wants to comfort suck but is getting frustrated as she is full and on the breast she is getting more milk than she wants.
Sigh, it's a tough world when you come without the instruction book, just hang in there and try a few changes to see what works for you and her. Good luck.