Vomit and not pooping

My LO is 8.5 months She’s on solids (mainly puree) but won’t eat as much and does not like to drink water. Milk is both breast milk and formula. I noticed since last week that she goes for a poo every two days, the first day she’s fine the second day especially at night she gets really fidgety and wakes up every hour or so, sometimes crying in her sleep. Last Thursday (day 2 of not pooping) morning she had her morning milk and about 30mins later she was really grumpy and fussy, she vomited the entire milk she had after that she was absolutely fine, had some solids later in the day and had her milk fine. Friday she did a decent amount of poo. Saturday and Sunday didn’t poop, last night again she was crying in her sleep, very fidgety, this morning she looked a bit tired and grumpy but was playing and laughing as normal. After her afternoon nap she was crying again and seemed as she was in discomfort but wanted her milk, she had 60 ml of milk and a bit of fruit about 30mins later she was pushing really hard to poop, whilst she was pushing she did another big vomit, she vomited and pooped at the same time. After that we gave her a shower, played for a bit and had her nigh bottle and fell asleep. She hasn’t moved at all, or cried. Touchwood she’s fast asleep. I don’t understand what I’m doing wrong, am I giving her something making her feel sick ? I’m giving her what she’s been having since she turned 6 months old, only occasionally I try something new and if she likes it I try again. Or is it because she’s not drinking enough water? Sometimes I can hear something coming back up her throat but she won’t vomit sounds like reflux. Feel terrible 😢 poor baby