Weird periods

I've had pretty normal periods (4-5 days, all around the same time of the month) since getting the iud (paragard) removed in February. Well this month and last (July and August) my periods have only been TWO days long with some spotting the next two to three days after. I took two pregnancy tests before my period in July and two before my period in August and they were all negative. What could be going on? Should I test again?