Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Some amount of fluctuations in the menstrual cycles can be seen depending on many factors like weight gain, psychological factors, hormonal changes etc. In your case the possibility of pregnancy is less as the intercourse was protected, you got four menstrual cycles after intercourse and urine pregnancy tests give negative result. And yes, sometimes women get bleeding as if in periods though they are pregnant because of incomplete obliteration of the uterine cavity with small gestational sac. If you want to be sure, please go for blood test for pregnancy once. If it shows beta HCG level less than 5 Milli International units per ml, the possibility of pregnancy is almost ruled out. Regular physical exercises like brisk walk can help in reduction of weight on regularization of menstrual cycle. Hope I have answered your query. Let me know if I can assist you further.