My child is having a hard time going poop?

my 2 year old wont poop at all today, but we have gone through so many pairs of underwear today bc she keeps getting little skid marks. its as if she has diarrhea, but yet nothing in the toilet. her hole is so sore from the little bit that comes out into her underwear. waht is wrong with her? what should i do....is there anything to help her go?