Adding notes changed time signature???

I'm writing in a score in 6/8, and whenever I add an extra note into a measure, it adds it into its own spot in the measure, making that specific measure 7/8. Adding more notes gradually brings up the top number of course. Did I accidentally enable a feature that does this or is this just a bug??? All my other scores work normally so I think it might be something I did in the specific file for this to happen.