Yeah, most likely it's rosin core solder.
You can, however, get a tub of rosin flux +alone+.
That stuff can be a life saver if you're trying to solder to some sort of metal that won't normally stick to solder, like, say, battery terminals, or chromed bits.
Smear a little rosin flux on it, then solder and it sticks perfectly. Just fyi in case you find yourself trying to solder something where the solder doesn't want to stick.