Nipple HELP

Hey guys so I recently had a baby (3 weeks ago) and I have been breastfeeding. My question for you mommies out there is this:

My nipples randomly ache/throb/feel hard (don’t know really how to explain the feeling) when I am not breastfeeding. Like I will breast feed and about 1hr later or 30mins later my nipples will ache/throb/feel hard...

Has this happened to anyone???