If the brown periods are comming in time as expected, then they are quite regular. You need not take a pregnancy
test every time just because they are brown, unless the periods are delayed by 7 to 10 days. When the quantity of bleed is lesser, there isnt enough of it to trickle down as fresh red blood. Instead it remains within the passage (being too less) and gets altered over few hours to the brown colour. It happen normally sometimes, and mostly reverts back to red periods over a few months. If it continues beyond 3 months you may need further testing.