How early after ovulation can implantation occur??

Katie

So I had a positive OPK on Tuesday this week (had unprotected sex the following two days) and woke up last night to cramping in the middle of the night. I am not due for AF until the end of March but I am wondering how early implantation cramps can come. I do cramp a couple weeks before AF semi regularly but I’m wondering if I may be pregnant. Have any of you experienced this?