lighter than normal flow, negative test

Usually my first day of my cycle is very light, brownish in color. Days two and three are moderately heavy. Day four is mild and 5 is back to light. 
This month, it has been the lightish brown color and not even needing a tampon for now the fourth day. So I took a test. Negative. 
Any thoughts as to why my flow would be different this month? 
I'm going to start to use OPK this month to help. 
Any thoughts, suggestions, tips would be great. Thank you!!