You seem to be having post coital bleeding (bleeding after sex). This can be because of local causes i.e. polyps, cervicitis
etc. You also informed about bleeding starting 1 week after stopping of menses. You need to have a gynaecological examination to rule out any causes for local bleeding. If it is due to certain infections, you can get treated. Sometimes ovarian hormonal
irregularities lead to irregular menses. Any polyps in uterus
can also cause the same. Ultrasonogram and hysteroscopy
if required can clearly make out the cause after looking for local causes in gynecological examination.
Wish you the best!