While this isn’t common, it does happen. It’s usually attributed to semen. Semen contains prostaglandins (PGs), and PGs cause uterine contractions or cramping.
You can test this theory by using a condom with intercourse a couple of times to see if you still experience the cramping. The condom keeps the semen from the cervix, which should reduce reaction. You can also use withdrawal, removing the penis before ejaculation.
You can also try taking ibuprofen or Aleve before intercourse; these are anti-prostaglandin medications that can ease the cramping as well.
I hope one of these approaches works!