What Causes Occasional Clots And Brown Spotting Between Periods?

Hi. I am a 31 year old mother of an almost 2 year old little girl. My periods are usually always regular but on occasion they may be a day or two late or a day or two longer.
I very rarely experience cramping during periods.
I have been having an issue now however that has got me concerned.
I skipped my period december 09 and i hinestly thought i was pregnant as i was quite active with my partner during my ovulation time, but after numerous preg tests i was not and my period arrived exactly when it was due in January. However since 7th Jan i have had on and off bleeding ranging from brown to pinkish and red. Majority of the time it has been brown. I have just again got my period on the 22nd feb a little later than it should be but it is here. Again no cramping or pain just bleeding this time with the occasional clot. I am concerned as i had this happen the year i fell pregnant with my daughter and my doctor told me it is normal to have you period go out of synch from time to time. And here i am again witht he same thing 2 years down the track please help???!!