Contraceptive pill missed period

My girlfriend has been on the pill, and the last time we had sex was more than a month ago. After which, hHer period came 5 days later but was a bit light. A month later(now), she's got some tender breasts and her period hasn't arrived (4 days late).

She's never missed a pill, and has always been on time ( the most was 4-5 hours later than her usual time).

Could she possibly be pregnant?

Even if the pill failed, her first period a month ago would rule out her not being pregnant right?