Yellow is the one color that is pretty elusive in the geranium world - so-called "yellow" pelargoniums are not really yellow at all but a creamy white, often with a greenish or very pale yellow tinge.

I suspect your mystery plant is not a geranium at all. Caltha palustris seems like a likely possibility or it could be one of any number of annuals. But without a photo, this is all pretty much just a stab in the dark.