What Causes Delayed Periods?

i had sex with my gf on 15/03/2015 with male condom as precaution and last her period occured on 2/03/2015 but it has been 26 days she is not getting period while its generally 25 days or so, we both are worried as what to do. Please guide us what test can be effective in diagonising if everything is safe or not?