The other day i thought i started my period....?

but i guess it wasn't. When i woke up i spotted really dark brown thats how i knew. well Now i am not even spotting. I don't know whats going on. I really wanted to have it cause i am flying in a week and don't want it then. I have had problems with my period cause it's very unusual. it comes every 3 months now but stretches out by 2 weeks each time. I went for a sonogram but the doctor said i am fine. What do u think is wrong?