Need some advice about my kittens pleases asap?

welll i have two 7 week old kittens and when i got them i put them on felix kitten food. since feeding on them they have been fine but when they started to poo in their litter tray i noticed a dot of blood in their poo but then i took them to vets and they said it will be their stomach getting used to their new food. so a couple of days later which was yesterday 1 of them got diarrhea which w really runny so i got told to change their food to the I.ams (how ever you spell it) dry kitten food so they have been eating that today and their kitten milk and water. what is the diarrhea off and the blood..!!! the woman i got them off said she wormed them the day i got them and i have had them 1 week know.!!!! but she Said she had them on food for a week but her friend told me She only fed them kitten food the day i got them So really they have not been weened properly.but 1 of them likes to Suck my finger n when i have my dressing gown its like he tries to look for the mother teet which i think they Should of Stayed with their mother an extra week :( help pleases x