One day of light menstruation and the rest dark brown VERY light

My menstrual cycle is always on time and usually last 5 days but my last cycle which started 2/23 only was light for not even a whole day. It typically is very heavy the first 3 days and then becomes light but still pink. This time however, it was very light pink and started in the morning. By the time it was night the color turned from light pink to dark brown. It was been a week and it is still dark brown but very light.
What could be the reason for this?
I am a freshmen in college and I have been under a lot of stress having to drop out of a class today... I also have recently started working out the last two weeks and wonder if that may be the cause... Another factor may be that I am sexually active with my fiancée and it just may be that I am pregnant...
I am confused and completely stressed out...
Any suggestions of what it may be or what I can do?
Please.