Cramping days 18-19? (30 day cycle)

Pat
We have been TTC for 13 cycles (currently in cycle 14). I ovulate every cycle and have a fairly regular cycle of 29-31 days. I usually ovulate day 16.
​On day 18 I had mild period like cramping in and off and it was moderate cramping on day 19 most of the day. Today is day 20 and the cramping has stopped. I haven't had any bleeding and I don't cramp anytime (usually!) other than 5 days into my period.
​What could it have been?