It is practically impossible to get pregnant from a toilet seat, though I recommend visiting your primary care physician
to discuss this concern. The sperm in the semen are designed by nature to survive in very special environment. These sperm are delicate to temperature and conditions. When outside of the body the sperm will dry out and die within moments of being exposed to oxygen. Even if a woman deliberately tried to introduce some of the semen into her vagina, survival would be unlikely. To improve the chance of a conception the woman's vagina would have to make sufficient contact with the wet semen where sperms penetrate the vagina. Simply put, ejaculation must take place inside the vagina. The sperm never hit the outside heat and they swim up the fallopian tubes to fertilize the egg. In addition, it takes several millions of sperm to make a woman pregnant even though one gets in to fertilize an egg. Most would die long before reaching the fallopian tubes if they had to travel all the way from the beginning of the vagina. Thus, the chances of an "accidental" conception under the circumstances you mentioned is practically zero. I would suggest a visit with your primary care physician to discuss more in details about how to get pregnant and sexual transmission diseases as well as how to prevent pregnancy and not get yourself in the situation where you will need to worry
about if you will become pregnant.