Cause For The Delay In The Periods?

Hello I am 4 days late on my period I have no sign well sometime I have stomach pain and sometime I feel sick. But I though it was because I took diet pill because that when it start hurting and feeling sick. But a week it came back again. My last period was march,13,2014 I had sex between 26- around the end of march. I normally get my period the first or second week of each month. I having got it in April or May .