Thank you for your query.
As you mentioned, you are not on any birth control, so could you please specify what drugs you have been using to ensure that you would not get pregnant ?
ALso, ejaculation inside the vagina would not cause any menstrual irregularity.
You have just had an irregular period.
hence, you should rule out a missed pregnancy
, or an ectopic pregnancy.
Cervical lesions like growths or polyps, endometrial pathologies etc also cause post coital bleeding.
Possibly the drugs you have mentioned could have caused such a side effect.
Please visit your gynecologist for further clarity