I had protected sex with my girlfriend on December 21st. Usually her period is on the fourth of every month. She had light bleeding that lasted about 30 minutes on third of January. It has been almost 44 days since her last period. Can you please help us to know the reason?
"Protected sex" only reduces the chance of pregnancy. It cannot eliminate it, just as it cannot completely eliminate the spread of diseases. For example, using a condom reduces the odds from 85% to 20% over the course of a year. 20% isn't anywhere close to zero. This doesn't mean she is pregnant. It is possible for other reasons that she just happened to have a light bleeding this month. However, to be sure she needs to use a home pregnancy test. Generally they recommend testing two weeks after a missed menstrual cycle.
Regardless of whether she is or is not pregnant, you two have been violating God's laws against having sex prior to marriage. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10). Faithful Christians don't go having sex with their girlfriends. They have too much respect for the girl they love to use them for personal gratification. Instead, they commit their lives to the woman they want by their side for the rest of their lives and then sex follows the marriage.