Covid vaccine

My daughter got her second Covid vaccine (moderna) at her 9m appointment (as well as flu shot and hep-b). The next day her poop was black and more liquid like diarrhea and has been this way for 5 days now. Pediatrician’s office says that happens with the Covid vaccine sometimes. I trust them, but want to know if anyone else has experienced this with their littles? It’s been a nightmare with daycare not wanting her to attend with diarrhea. We’re gonna go into the doctors office later today to try and get a doctors note and make sure nothing else is going on. That’s literally her only symptom though. Otherwise, she’s completely normal and happy.