The immediate answer to your question is: yes, horses, just like people, have good days and bad days.
However, your mare suddenly getting aggressive kind of makes some red flags go up. Something could be wrong... Pain is the #1 reason for horses getting aggressive. Does something change on those days?
Also, how long has this been going on? This makes me think that she may be in heat... some mares get really sore and grumpy when they're in heat – their sides get tender and they ache – just like people. Hormones are also a very likely cause.
There are supplements for mares that I think you should try. Since most of my horses have been geldings, and those that were mares didn't have any problems, I've never used one before. But I've read good reviews on Mare Magic, SmartMare Harmony, Mare Plus, Easy Mare, and RelaxHer Blend. I really think that you should give one of these a try, just to try to help her. SmartPak has a great supplement selection, so if you need a place to buy these, there's your place.
Also, horses lick each other when they're lacking something in their diet – most likely salt and minerals; the horse next door should be given a salt or mineral lick.
Hope this helps – Let us know how it goes!
The hills were bathed in moonlight, the shadows not so stark;
Silver light reflected off his brown hide as he held me in the dark
– I love you, Mudpie!