What Causes Delay In Periods?

Hi..I am 29 years old..my period is normaly regular expect for one time when we got married in march 2010.. we are trying to have a baby..my last period was on the 23rd of april..and now i am delayed for about 4 days..took two home pregnancy test and both were negative.. can i be pregnant..i have used some cortisole cream n voltaran for about 3 to 4 days for itching and musle pain from 16- to 20th may..can this be a reason for a delayed period..thank you...