lost a gallon of beer in fermentor?!

I posted this here cuz im not a smart person but i recently brewed a hefeweizen partial mash style..i compensated for grain absorption and evaporation during boil. I had a final of 5.5 gal in fermentor pitched my starter and after aggressive fermentation was over i peeled and quartered two oranges and pitched into fermentator. Well theres a heat wave (according to Pacific Northwest standards) so I moved the Hefe to cooler parts of the house. Well upon lifting the bucket i nearly jerked it to the sky !? So i opened it up and I apparently lost 1.2 to 1.5 gal ? WTF ? Help me HB GODS...help me understand where my wonderful beer went?

what was the temp of the brew? i had the same problem with a Belgian Golden Strong i made. i ramped the fermentation temp up to the low 80s and by the time fermentation was over i had way less than what i started with. evaporation out the airlock. either that or gremlins drake some of my brew.