I got [prefix] [suffix] [infix] from washing the [whole].
P.S. I [infix] the [whole].
The note above, when the gaps are filled, will be a grammatical and reasonable note.
What is the note?
It is not stated in the question as it stands, but if it is legitimate to assume that the two 'infixes' mentioned in the riddle need not be the same, then here is one solution:
I got WATER... ...ON ...ME... from washing the WATERMELON.
P.S. I ...ATE... the WATERMELON.
Here, WATER is the prefix, ATE and ME are both separate infixes within the word, and ON is the suffix.
Of course, nowhere in the rules does it say that every letter within the whole word is used in this puzzle. This is pertinent here since:
Nowhere in the 'fixes' is the letter L clued. One (slightly tongue-in-cheek) way to overcome this is to assume that while washing said watermelon our friend MEL got a little too close to the tap and ended up being sprayed, meaning that:
I got WATER... ...ON ...MEL(!)... from washing the WATERMELON!