My other thought is that the lycra in the old bras was stretched out so you didn't notice the changes in your breasts. How do the cups fit under the arms and do you get fat showing in the back? Some sources I've read state that back fat is actually breast tissue that has migrated. As thin as I am I had the back fat but don't notice it as much now that I'm wearing better fitting bras. But spilling out the side of the cups is also a sign of a too small cup.

Until you know your size in a brand order 2 sizes, or 3, in a model and return the ones you don't want. For example, if your band size is 34c but reviews say the band is running small you could order a 34c and a 36b. There is a chart on the HerRoom site that explains how underwire cups change as the band size changes. They also note what cup size to use if you're going up or down a band size. If you find yourself ordering a lot of bras that don't fit then your in the wrong size. When I was ordering a lot I combined orders when I returned them. Saved me on shipping.

Keep in mind that some brands tend to fit narrow breasts will others fit wider ones. Panache Cleo works bests for me because they tend to have a wide center gore, whereas, Freya bras have a narrow center gore. The bands on Freya's are very stretchy so this is a brand that I could use a 26 band in.

Don't get discouraged. Bra fitting, whether online or in a store, is hit and miss.
Once you find a brand that works for you stay with it.

There are also a lot of blogs that explain bra fit and often review bras.