Irregular period twice a month.

Dr I want to ask u that I have facing irregular period issue from 2 3 months . From 2 months I have my periods twice a month my periods date was 19 20 and 10 11 last time my periods was coming on 19 and 20 August and after that on 28 August I m done protected sex and I checked thrice from home pregnancy kit it is negetive and I experience no symptoms of pregnancy but my periods was delay according to my calculation the date is 10 sep but I got no periods till now. Can u tell me the reason and suggest me some tablets I was know about meprate tablet for irregular period but I don't know about doses can u tell me the doses and also that what's the reason behind delay period after I m done protected sex and result also negetive