Breakthrough bleeding

Hi guys,

I’m on the pill and I have been for about a year now. I haven’t had any problems until last month when I missed a pill and then a few days later I started spotting. A week later when I got to my sugar pills I had my period, and then once my period finished all was back to normal for a few days. Then today, the spotting started again but slightly heavier, despite being back on active pills. What does this mean and how do I treat it?

ps I’ve only had sex once in my life and it was about 4 months ago, the only protection we used was the pill. What’s the likeliness that it’s an std??