Some guys have very sensitive nipples, by itself that really means nothing. (some guys find this worrisome, others find it pleasant)
Get three measurements:
you'll need help, as all will need to be taken with your arms relaxed at your sides.
1. Upper chest, as high up as you can measure straight around your chest with your arms cown at your sides.
2. Nipple line, over the fullest part of the breast. the tape should be just tight enough to stay up and not tight enough to distort the breast.
3. Lower chest. around the rib cage just below the breast.
The increase in chest measurement could be due to increased weight or possibly due to muscular development.