She obviously got a taste of what it was like to make love regularly and is now disappointed that things have changed. Couple that with your regular rejection and she probably feels unwanted, unattractive and sexually frustrated. Thats a horrible mix.. especially as a hormonal girl. Even though its not commonly discussed.. men are not always the virile sex orientated beings they're made out to be.. things like stress, tiredness and underconfidence can affect libido and desire. Yes you may be tired but are you sure you are not making an excuse? You make it sound as if making love to her is a chore and something you do because you feel you 'have' to.
Giving you the silent treatment isn't necessarily mature but she's probably resorted to this because she feels so bad about this area of your relationship like her said rejected etc. Also the whole telling her to take a shower thing (whether its in the morning or after a sweaty episode in the garden) probably killed the moment for her. Next time she isn't up to your desired cleanliness standard why not jump in the shower together and start proceedings from there.. might be a lot less offensive and a bit more enjoyable that way.
She is trying to initiate a response from you with the silent treatment.. so I'd suggest having a proper chat about it with her.. listen to how she feels and tell her how you feel.