I'm 18 not pregnant as got my regular menses one week after intercourse nor feeding. But noticed a recent increase in my breast size. Why? Is it normal

Weight change? Breast size can fluctuate from hormonal changes, water retention, weight gain. You mention you noticed this after intercourse which makes me think you may be concerned about pregnancy. If you think you might be pregnant check then check a home pregnancy test. They are cheap, reliable and available at any pharmacy.