You may find it weird, but we sometimes use "double diminutives" in colloquial speech Probably it's the reason of your confusion. Such forms are not found in any dictionary, they are very colloquial; you can only recognise them by their diminutive suffixes.
Ручеёчек is a good examples. Others are денёчек (день -> денёк -> денёчек), ножичек (нож -> ножик -> ножичек) etc.