Spotting instead of period on the pill

I’ve been on the pill for around 3-4 years and nothing changed. I always get my period on a Monday or Tuesday on my packet during my placebo week, and I always get a heavy flow of blood and such. But this time I didn’t. I spotted Monday on 24th, and had my usual symptoms which is lower back pain and shooting pain up my butt lol so I thought ok my period is coming. Today is Wednesday 26th, and I’m supposed to take my pills again tonight. But all this time I have not bled red heavy blood, only brown spotting and barely tbh. I’ve had unprotected sex this month but I’m consistent with my pills, never missed one. I’m curious does anyone know what this means? This has never happened before.