Triggering period?

Julie
I'm 4 months post partum on the nose today and have started noticing a trend. 2 days after sex it my period starts again. 
It started at 2 months having 2 periods that month, 2 periods in month 3. Before pregnancy I was on a regular 28 day cycle.  
Anyone else heard of sex possibly being a factor? Or experience your cycle being extremely altered?