What size and frequency of milk feeds are you giving them?
At that age it is very unlikely they will be eating enough creep to cause any problems at all, and they certainly still need their milk for at least another 3-4 weeks.
Is it possible they have an e-coli infection? It's so hard to keep everything clean when bottle-feeding 3-4 times, or more, a day, and they pick up infections really easily. I usually find that mixing a tablespoon or two of live probiotic yoghurt into a litre of milk for a feed or two, and being extra specially careful about keeping everything very clean, clears them up.