Bra fitting 101?

Cat
This morning I woke up with my boob like...out of my bra lol. I usually wear a 34a but my mom got me this 34b. When I looked at it on me, it actually looked too small! Yet I wake up with my boob spilling out. And still yet I usually wear a smaller size! 
When I measured myself, the fullest part is 32.5 and the band part is 29.5...I put this in an online bra calculator and it recommended 32c...
So to recap
Usually wear 34a
Spilled out a 34b that looked small on me
Recommended 32c by bra calculator using 29.5 and 32.5
Help!
Update: other calculators tell me 30c, 34c, and another 34c....if I truly am in this range somewhere then how have I been fitting in an A cup :0