My 4 mth old baby ping stool contains black tiny threads like stuff, stool color is yellow green.?

Hi, can anyone ans ? My 4 mth old son having fever & ping small black tiny thread sort of stuff in stool & colour of his stool is slightly greenish, we r giving him Aptamil & feed. We gave him nerofen 1 day for fever & next day we observed this threads sort of thing in his stool. I'll b thankful if anyone explain what is it ? Thanx