My horse does this sometimes - she just completely leans against the contact, and puts her head down really low. The main thing to remember is DON'T let her pull the reins from your hands, in my case anyway, she does it to try and evade the contact and get more rein for herself.
When my horse does this I just push her on with my leg, and half halt the reins to bring her head back in. I find this helps me a lot, because you will find that as they get more impulsion, they automatically tend to lift their heads up.
I wouldn't know exactly why she was doing this to you, but I would guess that she just wasn't in the mood for work - the conditions weren't great, and she'd had a week long break, so she probably just couldn't be bothered to work properly.
Hope that helps a bit.
Whoever said money can't buy happiness didn't know where to buy a horse.