Irregular periods. Birth control pill.

My age is 37 year old . My baby is 2 year old . For very first time in my life I missed the periods in feb . I got periods on january 19 . After waiting for so many days . I took meparate 10 mg twice a day from 27 feb to 2 march on consultation with family doctor . On 10 march I got periods . I know it is withdrawl bleeding . Now next periods when should i expect . I understand we cant predict the date but still I want to know since I dont want to depend on medicine for inducing periods . I want natural periods whether If it comes on interval of 37-40 days also . Anyhow we are not planning 2nd baby also . Can I wait full april to get periods . Or upto how maximum I should wait before going for Thyoid / prolactin test or ultrasound . Is there any medicine to regulate periods like duphaston etc . It is first ever case of irregularity since my menarche . Even when planned for baby , I conceived in 1st try also . It was natural conception . I never had any birth control pill.