Why is my "period" still on?

Okay I am 19 and have had a period for about five years now.  I usually always have slight brown discharge before my period starts and after it goes off.  Only once have I had a straight brown period.  My period is irregular and has been since I got it.  But usually it lasts about 6-8 days including the brown discharge.  This month (december 2010) my period started on the 6th with the brown discharge.  It got heavier as normal but then it never turned red.  I wasn't worried since I am irregular and I've had a brown period before.  So it was going off and getting lighter.  Then last thursday, the 16th it started getting heavier again.  It's still on now, the 20th.  It's not as heavy as a regular red period flow.  It's like large clumps of brown.  I have no idea why and no on google has had the answer for me.  Please help.  Thanks.

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