Infection can appear to be sperm. All sperm plugs that I've seen are more solid than liquid. If it looks like an ejaculation, then it's probably not a sperm plug.

I'm not sure, but if it continues, a vet visit may be in order. If it's an infection, you may have to work with the vet (usually means a lot of money) to find out the cause.

Good Luck!
Jim Smith