Okay this might be confusing

Okay i been ttc since dec took my iud off in august 2014  , i always had normal periods, my period where normal after the iud but they got shorter after, comes to find out i have a cyst and the doctor put me on bc to try and see if itll go away, well anywho after i was done with the bc he said i should get my period which i thought i was getting but i just spotted for like 4  or 5 days superrrrr light, this has never ever happend to me and i know its not pregnacy cuz i took all the bc except for one day that i missed, could this just be cuz my body is not use to takingbbc pills.??? Im scared cuz im also takin diet pills but im starting tt