How do I know if it's diarrhea??

Jess

My baby is 4 months old and until now has usually pooped 1-2 (maybe 3) times a day. The past two days she's been long 4-5 and they look waterier (they seep into the diaper more than just lay on top .

She's eating normally, and lively as ever so I don't think she's sick. But how do I know if it's diarrhea? Everytime I change her diaper lately, there seems to be a new poop.