If your bra is uncomfortable, it's likely you're wearing the incorrect size. A well-fitting bra should never be uncomfortable to wear.
If your bra is too small, you might notice the bra cups are cutting into your breast tissue, the bra band is riding up in the back, or the bra is leaving red marks and hurts to wear.
If your bra is too big, you may experience your bra moving when you move, gapping at the cups, or slipping straps.
Another reason your bra may be uncomfortable is because it needs to be replaced. Some signs it's time to replace your bra are:
The underwire has poked through the fabric and is stabbing you
The band has stretched so much that it feels loose on the tightest hook