Take her to the Doctor. Its always best to check.
Sometimes, they get gas build-up or constipation... but if you don't know for sure, you have to take her to the Doctor.
babies/kids don't always poop everyday. So you can't really go according to her frequency either.
Does she have pain, is fussier, have other symptoms, having a hard time eating or drinking?... there are so many things you need to look out for. AND it can be anything. But if you don't know what to look for... then time is passing and she may be in more discomfort.
Don't second guess it. If it is something else, you don't want to regret not taking her to the Doctor. Also, you don't want to "guess" and give her over-the-counter things just "guessing" at it either... what if it makes things worse or not?
*Also, NEVER give young children/babies Pepto-bismol (that pink stuff) or other "adult" things for upset stomach or gas... it's not good for them when young. Something in it.
You should ALWAYS ask your Pediatrician first.
Give a call in to the Doctor... during "off hours" if you call the Pediatrician's office, their phone recording SHOULD say who and where to call in case of emergency... or the Doctor will have a pager/voice-mail. I would call them right away.
Take care,
Susan