i normally have a very healthy, normal menstrual cycle. last month, i took a plan b, and my period was about a week early, and it was very light; it lasted about three days, and i only had to use one tampon per day it lasted. i assumed that this change was due to the plan b. this month, however, my period is again early (its been about 21 days since the first day of my last "period"), and it looks again like its going to be pretty light. i also usually have some cramping on the first day of my period, but these last two periods have had no cramping whatsoever. (im not sure if thats a good thing or what) also, i stopped using birth control in january, and started on an antibiotic called mynocyclin for acne. im 19 years old, and confused as to this new early and fairly light period. there has been no spotting in between. should i be concerned? is this abnormal? is 21 days in between light periods normal? please someone help.