Hello! First I want to say congrats on getting to 5 months!  and second, congrats on choosing a wonderful pediatrician who has given you very good information of no cereal until 6 months! I commend them!  So... to get to it, there could be several things going on here. and my guess is, it is NOT hunger. If she has done well on breastmilk alone, until now, then your breasts have done an adequate job of taking care of her.  Because you have introduced formula and she still is acting hungry, that also gives me the idea that its not from hunger.  Breastmilk is done on supply and demand, so yes, the supplementation will be depleting your milk supply  by giving her formula. But I don't see the need to supplement if she is acting hungry after that as well. So... my guesses would be... shes is going through a growth spurt. which babies go through at 6 days, 6 weeks, 3 months, 6 months 12 months and 18 months... some a little earlier some a little later depending on baby. when this happens , they act starving no matter how much you feed them because they are building your milk supply in your breasts ( supply and demand ) and then after a week or two it will slow back down to normal again.... second idea is shes teething. and shes not really hungry but acting a little confused as they find nursing is comfortable and releases endorphins ( natural pain killers ) so she wants to nurse all the time to hinder the teeth issue she has. she may be teething long before you see teeth.  another idea I have is that developmentally , every 6 months, every baby goes through a developmental brain growth. which often takes them to a fussy place, they become children that are clingy, fussy, hard to handle, challenge on everything, its like a mental emotional growth spurt so to speak.  where nothing you do ever seems right and melt downs ensue quite often. You will notice this every 6 months. SO when you are periodically saying to yourself, " i don't know what has gotten into my wonderful sweet child, shes gone off the wall and I want my sweet child back... notice if its around her birthday , or her half birthday. and you might see the significance...

So those are my suggestions. I hope that was helpful. I truly don't think its a hunger issue. if shes been thriving well, Breasts don't just suddenly stop working. If you need further detail feel free to write. there couple be other emotional stuff going on as well. But rest assured, you are not starving your sweet pea :)