Is it common to ovulate early?

I obviously know that's it's possible but I don't know if I did or not. So my cycle is normally 31 days long and I'm currently in my fertile window. I've never track my ovulation so I don't know when I normally ovulate. I had sex 3 times over the past 2 weeks. On Sunday I was cramping in my lower abdomen on my left side. So maybe that was implantation. I think that because the last 2 days I've been crazy tired like way more then normal and my boobs have been hurting so bad and so swollen which doesn't usually happen until closer to my period. Either way it's fine I'm just curious if anyone has experienced this.