Help I have a mono boob !

Breasts touching each other ? Rubbing against the ribcage ?
Check the front of the bra. I can bet that the front of the bra doesn´t like flat on the sternum. Instead, it is making a bridge between the breasts. This is how the breasts look from the top:
The water often symbolises sweat, caused by the rubbing of the breasts.

The band doesn´t lie flat on the sternum because the band is too loose. Most people guess the opposite, and try bigger and bigger bands hoping the problem will go away... but the bigger band you take, the quicker it will climb up over the breasts and make a "bridge" (it is being pulled up by the bra straps).
When the band is snug, it will stay under the breasts, and lie flat. It will support the breasts from below, allowing the cups to lift and separate the breasts.
When you go down a band size you will need to simultaneously go up a cup size, because cup sizes are relative to the band sizes. Instructions on how to measure yourself correctly are here.

Check these out: