if my girlfriend uses toilet immediately after me?

Hi,
Welcome to ehealthforum,
Sperms can stay alive for hours in wet environment. If they got washed off completely leaving no traces of semen on the seat, you can relax. If they had been present, and if your partner has used the seat within few hours time, and if semen has come in contact with vagina (and if she had been during her fertile days/closer to ovulation time), chances of pregnancy are possible. Wait and watch for her periods. Look out for development of any new symptoms if pregnancy is suspected. She should visit her gynecologist for further evaluation and assistance if she does not get her periods within 10 days of expected period dates, or gets abnormal bleeding. If pregnancy gets ruled out and if you are not ready for pregnancy, but intend to remain sexually active, you should consider getting her started on suitable regular birth control measures to be protected (after her periods). She should be regular with birth control use as instructed. Avoid carelessness and follow proper hygienic methods.
Take care.