The copper coil is a very effective ( more than 99%) contraceptive
method. Moreover your pregnancy
test is negative. So it is extremely unlikely that you are pregnant.
If you have had home pregnancy test then get blood pregnancy test done as it is more accurate as there is a very small chance of the contraceptive method failure
Can you feel the threads of the coil. Do you have vaginal discharge? Do you have urinary complaints.
If the pregnancy test is negative then see gynecologist to make sure that the coil is in place and to evaluate for vaginal and urinary infection.
If no infection and the coil is in place take progesterone
to start periods.
Some women do get cramps just before periods and it is normal. The symptoms subside with the period.
I hope I have answered to your satisfaction.