Period in middle of pack

Hi! I've been on the pill for about 2 years now... Recently my pharmacist switched my birth control for this month because they were out on the original. I've done this a few times and nothing has happened. I had gotten busy with work and school and forgot to take my pill for 3 days... I started being moody cranky and cramping and now I've started my period. Is this normal? Am I pregnant? Will I have two periods this month? Is this "break through bleeding"?  I will use protection after this period and for the rest of the month and all pregnancy tests have said negative.. In just worried