What Causes Delay In Periods?

Hi doctor....i am 25 years old unmarried girl. my period missed in Nov. so that after 4 days i called my doctor and he prescribed me Meprete for 5 days (2 tab per 1 day) along with Orofer xt and M2 Tone Syrup. After 5 days i finished my Meprete tab and also continuing others two which is prescribed by my doctor. He told me that after 7-10 days again i got my period regular in time. I also faced my irregular period from a long period of time, but this is the first time it was 4 days late so that i also test Pregency test in my home, and also it was negative. But now it completes 7 days after taking Meprete tablets but till i dont have periods. Please suggest me what can i do....? pls doctor pls help me. i m very tensed about it.....