I Am 54 And Have Had Irregular Periods For About

I am 54 and have had irregular periods for about 2 years. I last had unprotected sex four weeks ago. I have been feeling weepy. I have not been pregnant since I was 21 but feel I am. I did a pregnancy test a couple of weeks ago which was positive. I immediately phoned NHS online who said that as pregnancy was so unlikely, I should see my GP in case it could be cancer. I had a smear 3 weeks ago and it was normal. I am currently taking 20 mg Citalopram.