HI I HAVE A CAT THAT IS ABOUT 9 MONTHS

hI i HAVE A CAT THAT IS ABOUT 9 MONTHS OLD HE HAS NEVER BEEN DE-WORMED AND HE WAS A STRY AT 4 WEEKS BUT JUST IN THE LAST 3 DAYS i SEE BLOOD IN HIS STOLE BUT i HAVE ALSO CHANGED HIS FOOD FROM DADS TO PURNIA CAT FOOD WHAT COULD IT BE WORMS OR THE CAT FOOD AND IF IT IS WORMS HOW CAN IT BE TREATED tHANK YOU