It may be, but then again fish get the odd spot and the next day it's gone.
But if you see more and more then yes it probably is, don't treat just yet but keep a close eye on them.
Water change as you mentioned is good, just make sure the water is same temp as tank you don't want to stress fish any more then they are.
Keep testing water you will notice nitrite kick in next.
You may like to do smaller water changes more often, say 5% a day to avoid the swings in your water chemistry.