Welcome to HCM.
Not necessarily it may be due any STD
It may likely to be due to tightness of your partner's vagina or due to lack of adequate lubrication in her vagina, some abrasions could have been caused over your foreskin during sex or it may be due to a mild yeast infection secondary to your partner's genitals.
If you could consult your physician he/she will help you in getting relieved.
It is a very simple problem. Nothing to worry.