Some advice about SEX AND MENSES

Hi, i'm 26 yrs and into a relationship since 1 yr now. Planning to get married with my partner. We had sex 2 times, first time unprotected sex and visited doctor just for safety reason, as i was scared, doc said the intercourse was before fertility period so not worry about. My second intercourse was with precaution as my partner used condoms the 2nd time . Just worried about my menstrual cycle as it irregular since 2 yrs now , i only get my menses for 2 day regular every month, i had lot of work pressure, family stress and my weight started increasing suddenly. is this the reason for lesser menses. And is it safe to have sex once in a months time. Will sex also increase my weight even after going for gym or doing daily exercises along with diet...