Has she been milking consistently since her previous baby has been weaned?
I assume that's pretty likely that's the case, but for all I know you may have dried her off since the last kid was weaned, so I have to ask to be sure.
If her udders look at all tight or differently colored to normal, or if you're getting this liquid inbetween normal milkings, or if she looks uncomfortable or the liquid she's expressing is a different color or odor to normal then it's quite likely to be mastitis, but sometimes when they've been dried off, either going into or out of the dry period, they express liquid like what you're describing.
If you think it's mastitis you would need to act pretty fast. What treatment you use depends on your beliefs, I'd personally use natural therapies but probably wouldn't recommend you try them first off if you're unfamiliar with them or mastitis. Some people would use antibiotics and follow a different milking pattern to cure the issue.
Best wishes.