Can a women who has had her period 3 weeks affter having sex be pregnant?

me and my girlfriend have had sex about a month ago. and i did not come in her but pre *** may have gotten inside her. i took extra pre caution by having her take the plan b pill to eliminate the chances even more or her getting pregnant because it is already a low percent chance to get pregnant from pre *** and taking the plan b pill would probably make it even lower. i had her take it on the second day after having sex. she had her period like 3 weeks later so i was convinced that she could not have been pregnant because she had her period. and let me point out that a women cant be pregnant and have her period at the same time or it would be a miscarriage. but then today she tells me that she has not had her scheduled period. but let me also point out that if a female takes the plan b pill that her cycle may change i think that this is the case. and she has already had her period so i think that she may be coming on her cycle late again. but she says she has feeling her breast swole but no other effects. i will take any and all questions people this is a serious situation.