You are suffering from the intermenstrual bleeding
and post coital bleeding which is omnious and needs full work up.
The most common causes include as benign as endometrial polyp
and genital infections and local lesions to as grave as cervical and uterine cancers.
Your ultrasound does not reveal anything.
In my view you should get a repeat Pap smear examination along with colposcopy
and biopsy if required and a diagnostic hysteroscopy
to rule out uterine pathology