Hello,
What you are having are irregular periods.
The cause of irregular periods could be due to hormonal imbalance, stress, chronic illness, polycystic ovarian disease, thyroid gland disorders or diseases, pituitary gland tumors or cyst, galactorrhea, prolactinomas etc. You should consult a gynecologist and get examined and investigated if the irregularity in periods persists.
Bleeding or spotting after sex is called post coital bleeding. It is most commonly seen in vaginal infection like Chlamydia, gonorrhea, vaginal inflammation etc. it could also be due to disease of endometrium that is lining of uterus, polyps (mass) in uterus and cervix, fibroid tumors etc. You should consult an expert nearby for internal examinations.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Anyone who has bleeding after sex should see their doctor or OBGYN. You can also go to a sexual health clinic in your area to have this investigated. It could be as simple as you having a naturally more friable cervix which means it tends to bleed more easily when traumatised. Your husband may be knocking your cervix during sex which leaves you with the post coital bleeding you mention. It can be a recent occurrance as your uterus could have shifted slightly after having children putting your cervix in the firing line, so to speak. It's important to have it evaluated because it can also signal the above issues that have been mentioned. Vaginal swabs and a Pap smear can give you more information.
Reasons behind your irregular periods have pretty well been covered and as with the above need to be investigated for your own peace of mind.