What Causes Irregularities In Menstruation?

Hello i hav been tryin for a baby with no look, i hav irregular periods bt am late by about 15 days i have sore nipples but breasts r not swollen, but i feel swollen around my belly wich my partner has noticed. I took a pregnancy test 2 days ago and it was negative. I feel very gassy as well. Please help i am very worried and scared i can not hav children. Thank u xxx