If you're doing under your breast, you should add 5 inches to get the total circumference. Make sure the measurement isn't over clothes, etc.
If you wore a 34 or a 36 before...how could you even put a 32 around you?
Are you checking for the cup or the circumference?
Do a check as well over the breasts, under the arms, that might be a better measurement.
Be professionally measured for sure....but, as well, maybe the brands you tried aren't good for your shape?
Triumph bras are a good fit, they come in sizes A - C. Playtex, Wonderbra, Warner, etc...all fit differently.
If you measure 32 under your breast...you are not a 32, that I would say for sure.
You could take the tape where you measure under the breasts, keeping the back where it is and slip the tape up above your breasts (not calculating by adding anything). You'd more likely be that size than the 32.
Then measure around your breasts at the fullest part....do the subtractions for the cupsize.
And, of course....the measurements are just a guideline because brands fit differently.
If your breasts are farther apart, Triumph or Playtex are good brands....if your breasts are closer together, wonderbra is a good choice.
Trial and error and frustration. Seems there's no standards, only guidelines.
But, in my opinion from what you state, you're definitely not a 32.