periods on birth control

I'm on my fifth month of birth control and have noticed my periods have been really light. The first month was light but the months following that are what's worrying me. 
I have been having sex since starting it and I'm wondering if the light period is because of the birth control or if there is a chance I'm pregnant? 
Before starting birth control I had a medium to heavy flow and it lasted five to six days and now I'm down to four with only one day that gets close to medium flow and the last day sometimes only spotting. 
It is still starting on time. It's never late.
Should I buy a pregnancy test and what brand would you suggest for an accurate result?I'm a college student so money is really tight and I don't want to spend money if I shouldn't worry about it. 

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