The most likely possibility is that you are having something called breakthrough bleeding
. Breakthrough bleeding is very common in people who are taking oral contraceptive pills, and it can occur for two reasons.
The first type of breakthrough bleeding occurs when the lining of the uterus builds up quickly mid cycle, which causes it to bleed. This most commonly happens when excess estrogen levels are present in the body, such as might happen if you just started taking birth control pills or if you recently switched types of pill. Generally, this resolves after a few cycles.
The second type of breakthrough bleeding occurs when the lining of the uterus gradually thins out, which occurs after taking birth control pills for a longer period of time. This "atrophic" lining is also susceptible to bleeding, and this type of breakthrough bleeding is more likely to occur in longer term users. If you think you fall into this category, then the bleeding is less likely to resolve spontaneously, and may require instead switching to a different type of contraception
or a different type of birth control pill.
Start by making an appointment with your OB GYN or primary care doctor
and setting up a visit. They will help you figure out what to do.