I predominantly breast feed my two month old but we introduced a formula bottle at 11pm at 6 weeks (the liberation is amazing). Her poo has always been yellow and fairly liquid though, without getting too descriptive around tea time, it usually soaked into the nappy by the time i changed it (which is straight away unless at night and we don't hear her going) so i can't really recall consistency. Since introducing the bottle, her poo is still yellow but is really liquid (think between double and single cream consistency) and there is a lot of it! It also appears less grainy than before. She is visibly uncomfortable before she poos, often for some time - not tears or crying but wriggling and straining and lots of red faces. She also wants to feed a lot when she's needing to poo but keeps going on and off and being unsettled on the boob. She then settles immediately after the poo and often has a big grin on her face immediately! If she's hungry she'll then feed properly, if not she'll just be back to my happy baby.
I feel awful as worried it is the formula doing this to her. We introduced it because she wasn't gaining weight well (she was a tiny baby from birth and struggled to latch for the first few weeks so lost a lot and her weight was being monitored) and often got tired on the boob at night despite being hungry still. I'm worried if we drop the bottle that she'll stop gaining weight and/or start feeding frantically during the night to make up for it, which she was doing before and left me exhausted as well as feeling like a crap mum who wasn't enjoying her baby's first precious weeks.
Is her poo normal? Is the straining normal?! If not, should I try another formula? Already on Aptimil which is meant to be the one closest to breastmilk. God this is really long - sorry, new to this! Comments really welcome.