The irregular bleeding could be for several reasons. Sometimes, a small benign non-cancerous growth called a polyp can grow inside the uterine cavity causing spotting. Sometimes the fibroid, even when small, can be growing into the uterine cavity and this too can cause spotting. A very easy test to determine if you have a polyp or a fibroid is an special ultrasound called a saline ultrasound where some salt water is used to fill up the uterine cavity, which makes for excellent visualization with ultrasound.
Another possibility is that you are not ovulating regularly. There are blood tests that can sort this out. If this is the case, the decision about what to do depends on your wishes. If you are not ovulating, and want a baby, then medications to make you ovulate can be given and are very effective. On the other hand, if you do not want to get pregnant, going back on the pill or another progestin-containing medication may be the best choice.