Aw mama, ang anxiety talaga pag ganyan, hehe! So a few of us here have mix-fed babies rin and honestly, it can be tricky kasi mas unpredictable ang pooping schedule compared sa full breastfed babies, some of our babies went 2-3 days without pooping and it turned out normal pa rin, pero mixed feeding does usually mean more regular poops kaya understandable na worried ka. The vaccines can sometimes affect digestion a bit, but 3.5 days is starting to push it especially for a mix-fed baby, so I think it's smart na dadalhin mo siya sa pedia this week just to be sure, better safe than sorry, di ba? In the meantime, try gentle belly massage or bicycle legs, and make sure she's drinking enough (both BM and formula).