Low blood count..please help!!?

ok so i have been feeling really tired lately and i went to my dr who did some blood the results came back that my blood count was low, he asked about my mestral cycle i let him know it was really light, he knew that i had been suffering from bad pains in my stomach which we thought were from my gall stones, he said that i am possibly bleeding internally and i have to go for more testing on monday...what could this be if its not bleeding from my stomach? and if it is from my stomach then what could it be and how serious??