The two most likely possibilities in this situation are pregnancy
due to improper contraception
and the same leading to inadequate preparation of the endometrium and thus causing a breakthrough bleeding
. To clarify, you should get HPT with a fresh early morning sample of urine. This will usually give you a definite result by now; if not, you can either repeat the test after a couple of days or go for the blood test
. Hope you find this information useful. Take care.