Kitten help! to do with worming and diarrhea/sick?

we bought a kitten friday gone and ever since we have had him hes had diarrhea and has been sick a few times the woman we got him of says she had wormed him a week today there is no trace of worms in his stools, the problem is i dont kno if i should take her word for it that he has been wormed. am thinkin his diarrhea and sickness might be down to worms hes eatin fine drinkin fine and hes very playfull. should i de worm him myself or wait afew days? also could there be anyother reasons why hes got sickness and diarrhea?