What Can Be Reason For Spotting When Its Not Periods?

Hi! I'm on Femilon prescribed by my gyanec since past 2 years. I'm 40 years of age. Recently I developed skin infection due to chemical cleaner and the dermatologist prescribed me Prednisolone 10 mg each (3 tablets a day) which were reduced to 2 tablets a day after 5 days. Now I'm on 1 prednisolone 10 mg a day. For past 3 days I have observed spot bleeding and it is middle of my cycle. Because of my BCP I know that my normal period would come after 11 more days. Other than that there are no other symptoms of anything else - apart from the fact that my legs are feeling weak - which usually happens during menstrual cycle but that is not even really noticable. What do you think it could be and how do I go about it?