period issues. someone help :(

ok, so i'm 19 and i've had somewhat irregular periods since i began having them. they've always been about the same in length and the flow is not usually different, but they just come on different days of the month. last september, my boyfriend and i had an accident and the condom broke. in a panic, i of course went to the drug store and bought Plan B. (was taken within the first 12 hours). my period DID come a week late or so, but it was only 2 days long, and it was not nearly as heavy as usual. i had researched plan b heavily (naturally!) before and AFTER taking it, so i wasn't too shocked to see a difference in my period. well, the next month my period came along ... still only lasting 2 days, and still not as heavy. i thought it could still be effects from plan b, so i didnt freak out too badly. well, long story short .... its now january, and i'm on the first day of my period, but the pattern of it being too light is continuing. i have the usual cramps and irritability that come with my period. i've taken 4 at home tests and all have been negative. i'm just wondering....what are the odds that i could be pregnant?  and is it possible that plan b has just really messed up my period? any advice would be really appreciated!
Answers:

plan b is most likely the cause. it's very rare to continue bleeding (having periods) when pregnant. birth control methods do tend to alter your menstrual cycle.


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