Periods with the IUD

I have had Mirena for 4 months now. For the first three months, my period was longer than normal (7 days when it was 4-5 before the IUD) and this month, I only spotted for two days beforehand and actually BLED for only one day. I know that Mirena can cause your periods to become nonexistent, but I just wanted to make sure that this is normal. Thanks :)