pregnancy while on injectable? is that possible?

Hi. I started taking injectable last year. My first shot was on Oct 23. I started to have bleeding/spotting in my first 3 mos and thought that was completely normal. I waited 7 days before doing sex after my first injection. The second shot was given to me on Jan 15. On Jan 18, my boyfriend and I had sex. I still experienced bleeding/spotting the following weeks. But later in February, it had stopped. I'm afraid I could be pregnant. But how? I strictly followed my schedule. My period is gone for like a month now and a half and Im feeling tired most of the time. I also frequently urinate. I have a 7-month-old baby so it's definitely not the best time to conceive again. Has anyone experienced the same? Thank you!