Hi,
At 4 months old your lab should have been vaccinated against all the common infections, including parvo.
4 months is very early for a lab bitch to come into season - you do not describe any othe signs to confim that.
Your vet must have a 24 hour service ( if you are e-mailing from the UK) and should be available in case of emergency. Describe the situation to your vet on the phone, and leave him/her to decide if it requires urgent attention.
Walter