Implantation pain

Help! Due to being ill over Xmas and then going to NY for new year, I didn't track my cycle this month like I have the last 5. When we returned from NY I did an opk straight away (which I though would be positive or if not the next day) according to my last cycles but I either missed it or I didn't do enough opk (3) Any way cut a long story short. We only BD 1 in what Glo says is my fertile week and for the last 3 days I've had a weird pulling/crampy feeling (not painful) just every so often, throughout the day, in my lower left and front side? Any ideas what this might be? I've not had it before and I'm due on Wednesday? Thanks