Chances of being pregnant?

Hey ladies! I've been on birth control pills for years and I've always done really well with it; I rarely ever forget to take a pill but even when I do I use backup contraceptives if I have sex. I messed up and forgot to take my pill on the 21st and then again last night, the 27th. (I know this is really irresponsible.) I had unprotected sex today, the 28th, as I didn't realize that I forgot my pill last night and he finished inside of me. I realize there is obviously a possibility but I read that it isn't likely to be pregnant unless you miss 3 or more pills. I was curious as to if any of you have had an experience like this. I was thinking about buying a plan B tomorrow. Suggestions or advice? Please no hate or negative comments, I realize I was really irresponsible but it was a true accident. Thank you all so much! xx