If she has just started an infection, penicillin may fix it this time. If she is beginning a cycle of internal laying or egg binding, it's only a stop-gap measure. Don't want to give you false hope. BTW, the dosage I gave you is a tad stronger than just the normal one. These reproductive infections are really awful and you have to give them the one-two punch if there is any hope of nixing that.
I don't know for sure what your hen's issue is, though. Sounds from what is coming from her vent that it may be loose stuff where it shouldn't be and that usually means infection sets up.