What Causes Missing Periods And Brown Blood Discharge?

i m 24 year old & my weight is 47 kgs. today is 18th day of my 1st IUI. i had done urine test on 15th day which was negative. till now i dont hv my periods. day before yesterday was my regular period cycle date. just now i have 3-4 drops of sticky & very dark brown blood discharge. till now i dont hv any feeling of periods. can i be pregnent or is it the start of my periods. i m prescribed to use sustain 400 2 times a day & difostan