What Causes Inconsistencies In Periods?

hi, i went to doctors 2 weeks ago gave blood and urine sample, when i phoned for results they put me on antibiotic in case of any infection that might be there, this antibiotic made me ill and caused my periods to start. Now i can bleed one day then nothing the next,this has been going on for 7 days now.i am on the contrseptive pill. why is this. they want more samples but havent said what they think is wrong why?