What Causes Delay In Periods?

Hi I'm somewhat worried because I haven't got my period this month & I was irregular in may I got 2 periods that month the 2nd time it came out very light & then in June I got it agian 23 days after the last period & I haven't gotten a period this month I'm getting worried. Oh by the way I went to the Dr the time I was very light because the discharge was almost brown looking & he said it was normal & we also took a pregnancy test! Should I take another? I'm just very nervous