The cause of soreness is most likely due to lack of lubrication or too much pressure. This can be alleviated by using lubricant creams,but, please remember that certain creams are harmful to the sperms. It is sufficient if you can have unprotected intercourse
on alternate days during the fertile period. Some precautions that can be taken are-do not move and lie in a supine position if you have an anteverted uterus and in prone position if you have a retroverted uterus
for at least half-an-hour after the act. You may visit a specialist for further suggestions. Good luck.