Child of 2.5 months of age should be given breastfeeding first, after if the child remains hungry then only give top feed. Formula milk should 60ml water and 2 scoop and should be with bowl and spoon and not bottle feeding. In bottle feeding, the milk drips on its own and child doesn't have to make an effort so child sometimes become sleepy and sleeps with an empty stomach. Also feeding at this age should be on demand ie whenever the child wants, not every 2 hourly so that child can take more quantity at one time. Lastly, if child is passing urine 8 times a day then not to worry, it will increase. Hope you checked at better weighing scale and with the same weighing scale each time.

Sorry to hear you are worried about your little one. Monitor and see how wait gain continues, if remains stagnant see your doctor. You have not mentioned frequency and duration of the breast feeds, and if there is enough milk supply? If less frequent then can step up number of breast feeds. With formula maybe she does not take over 60ml as has had breast feed with some milk already.