Suggest Medication For Irregular Menstrual Cycle

I have not had sex for two months, and last month my period came like always. This month it is 2 weeks late. I have been having a lot of allergy problems and have been taking zyrtec, but I stopped taking it because I started just always feeling tired. Then I started my allergy symptoms and I almost think that I was sneezing so hard that I hurt my stomach. I'm not pregnant, but why is my period so late and why is my stomach killing me!? Any thoughts would be lovely.