Why Are My Menses Either Missed Or Irregular?

i am a student. I often miss my period at time time i.e it never comes at time. recently I meet my boyfriend after 25 days of mens I have also take saheli pill for precaution, but still mens not come till date. it is about 6 week to meet my friend.I scared.Now I want to take any pill for abortion if it may or may not.