This is a dumb question but how do you know? My boobs feel uncomfortable in my bra now. It feels unbearable at work and I take it off as soon as I get in my car. I have to massage them after because they do feel painful in those bras. The straps dig into my shoulders and it's not that I need to put it on the loosest clasp. It feels extremely uncomfortable the size I have but the next cup size up is too big. I don't have big boobs at all, I've been wearing my current small size for a long time. But now it's so uncomfortable that I just cannot wait to take it off. Also, not all my boob is in the cup. If that makes sense. A lot of my side boob is just hanging out of the bra.
Yes, that means you need to get a sister size. Your boobs have grown, that's why they're spilling over and out the sides of the bra cups. Find a bra with a larger band with a smaller cup size. For example, if you're currently wearing a 34B, get a 36A. That's the sister size if you find your current bra too tight. A 36A should fit you better without the cups being too big, since a 36A is just slightly bigger than a 34B (please feel free to correct me if I'm wrong). You want your bra straps to sit comfortably snug on your shoulders (I suggest a cross- or racer-back if you want freedom of movement without sacrificing stability), and the band to sit horizontal below your shoulder blades. Hope this helps!
Of course you hate bras!! What a horrible device, like some Medieval torture device, for such delightful parts!! Sadly, necessary, lest they dangle, to your waist, in your later years! Bras should be comfortable, soft, and enhance your natural gifts!! You need to find a professional to get you into the right size, and cup!! You shouldn't be pinched, or hating it, wanting to take it off!! (Lot's of GFs and OK with shopping, gay, just loving ladies!!)
I'm assuming that you've loosened the straps? If not, then see if that helps any, but otherwise, it's definitely time for a new bra. I highly recommend going to Victoria's Secret to get fitted. I just did for the first time, and I was super nervous, but it turned out being fine. They will bring you bra after bra of different sizes and styles, and you'll determine what size and type you need. Maybe you need one without underwire? Maybe you're like me and have uneven sizes? I'm a C on one side and a D on the other, so I needed to get a bra that felt a little too big and tighten one strap to even it out, otherwise it hurt. You don't have to get a V. S. Bra though... Just take what you learn and get any bra you want.
Here's what a good bra should do: 1. Is your underwire sitting away from my chest wall? This means your cups are too small. 2. Are your breasts bulging out over the top, sides, or bottom of your cups? You should probably go up a cup size. 3. Is yoour strapless bra slipping down? You need to go down a band size. 4. Are your straps digging into yourshoulders? You need to go down a band size. 5. Are your breasts falling to the sides instead of going straight forward? Ditch the T-shirt bra for something seamed. 6. Is the back of your band riding up? Again, your band is probably too large. 7. Are your cups wrinkling or gaping? In this case, your cups are probably too large.
I find I'm a different size in different style bras and some bra styles don't suit me at all. Have you tried getting meaured and if you have room at the back have you tried going up a cup and down a band. When you initially buy bras they are meant to fit you on the loosest clasp at the back until the elasticity starts to decrease and you can use the tighter clasps. I hope you find a bra that suits you, it's horrible something which is meant to give you support is making you so uncomfortable!
You probably just need a different band size or cup style. A bra store (like Lane Bryant) should offer free sizing. They'll usually give you a range with two band sizes and two cup sizes that they would recommend, with the sizes changing slightly between styles (DD in a full coverage, DDD in a balconette). You should definitely get resized and buy some new bras.
You could try getting a bigger band size but you just have to go to a store and trying them on until you find one that's comfortable. Comfortable enough so you won't dread putting them on everyday and maybe comfortable enough that you forget that they're even there.