What Causes Irregular Periods With Spotting?

My period is starting on week three. It's been light to spotting as far as flow. My last period was in April and lasted 5 days. Before that it was I think in January. About 5 days. I've never had sex and I'm thirty, I weigh 176 and 5' 4". I'm not on birth control and my periods have always been irregular. I donate blood fairly regularly and am due to donate again on the 24th. Should I be concerned?